AAAAI–EAACI PRACTALL: Standardizing oral food challenges—2024 Update

2.1 Clinical factors

2.2 Surrogate markers

Food challenges are time-consuming, resource intensive and carry a risk of anaphylaxis, although there may be benefits to the experience of a food challenge including reported improvement in health-related quality of life^{14, 15} – something noted to be greater in those who self-treated with epinephrine following anaphylaxis at challenge compared to those who experienced more mild reactions.¹¹

The most commonly used and reliable adjunctive tests for diagnosing food allergy are skin prick tests and serum specific IgE levels.^{16, 17} However, these tests measure IgE-sensitization which does not necessarily imply clinical reactivity. Thus, interpretation of all surrogate biomarkers is reliant on a clinical history supporting the ongoing presence of IgE-mediated food allergy. Accordingly, clinical interpretation of “positive” surrogate biomarkers relies on either a suggestive clinical history of IgE-mediated specific food allergy, or a positive OFC when the ingestion history is unclear.

2.3 Case history

In the clinical setting, the case history is perhaps the most important component and the cornerstone for diagnosis, usually in conjunction with positive IgE tests. The case history should support characteristic symptoms of IgE-mediated disease, that is, symptoms consistent with mast cell degranulation and inflammatory mediator release occurring within minutes to 1–2 h of ingestion, except in cases of galactose-alpha-1, 3-galactose (alpha-gal) allergy that generally occurs within 3–8 h of mammalian meat ingestion.¹⁸ Patients with a history of severe anaphylaxis (i.e., required intubation or intravenous inotropes, or with neurologic compromise) have typically been excluded from interventional research therapies or food challenges because of the theoretical risk of a further severe reaction. Surrogate biomarkers are poorly predictive of reaction severity, so case history should be considered when undertaking an assessment of risks pertaining to food challenges for individual patients. It is largely known that self-reported food allergy exceeds the challenge-proven incidence suggesting that OFCs should always be considered in doubtful cases.¹⁹

2.4 Skin prick testing

Skin prick testing (SPT) may be performed using commercially available extracts or fresh food. Protein content within commercial extracts is not standardized, extracts may lack relevant allergens (Bet v 1 homologues, LTP, oleosins, etc.) and variability may exist from batch-to-batch and between manufacturers.¹¹ The type of SPT device used may also contribute to variability in results.^{20, 21} Fruit and vegetable extracts are likely to lose potency over time and prick/puncture testing with fresh fruits and vegetables, including use of different cultivar strains (e.g., apples) is more reliable when indicated based on the patient history.²⁰

Efforts have been made to correlate challenge outcomes with SPT results, particularly in the pediatric population; however, because of the inherent variability in SPT results, reported standards must be interpreted with caution. A positive skin test result is generally considered a SPT wheal ≥3 mm greater than the negative control; however, as mentioned above, this indicates sensitization only and does not equate to food allergy.²² In general, an SPT wheal ≥3 mm has low specificity and can have a poor positive predictive value, which may lead to over diagnosis. At least in children, a larger SPT wheal diameter correlates with a higher likelihood of clinical reactivity^23-26 while a negative SPT typically implies the absence of IgE-mediated food allergy. However, since positive and negative results are not 100% reliable for any food, the clinical history must be taken into context when considering an observed challenge versus home introduction. Given a diagnosis of peanut allergy, SPT wheal size does not reliably predict the severity of symptoms.¹¹

Efforts have been undertaken to improve the diagnostic accuracy of SPTs by use of single allergenic proteins to predict the OFC outcome and have been reported for several foods.⁴ SPTs are not able to predict tolerance or reactivity to baked-milk or baked-egg in children with allergy to raw/fresh milk and egg, respectively.^{13, 27} Thus, the OFC remain the diagnostic standard for establishing tolerance to baked-milk and baked-egg.

2.5 Serum allergen-specific IgE

Allergen-specific IgE testing is readily available; like SPT, its presence represents sensitization, and not clinical reactivity, that is, allergy to a food. Therefore, allergen-specific IgE testing is not a reliable surrogate for a food challenge to confirm diagnosis. Decision-points for some foods (e.g., milk, egg, peanut, hazelnut) have been proposed using the ImmunoCAP or UniCAP (Thermo Fisher Scientific, Uppsala, Sweden) system as well as by Skin Prick Tests. Table 2 is modified from the recent meta-analysis by Riggioni et al. on the accuracy of diagnostics for food allergy, which included 149 studies evaluating heterogeneous populations. Such meta-analyses enable the synthesis of findings from various sources, which then enhances the reliability and generalizability of the overall results.²⁵ Predictive levels established with the ImmunoCAP system do not necessarily translate to other diagnostic platforms.²⁸ Decision points established for milk and egg may vary by age, and even within the same clinical center.^29-32 Even combined with the clinical history, they provide only limited accuracy regarding the potential outcome of an OFC.

Component resolved diagnostics (CRD) measure specific IgE levels to individual proteins derived from a food, and aim to better identify clinically relevant sensitization compared with irrelevant cross reactivity; however, CRDs do not consistently predict allergic reactivity.^{11, 22, 32} For example, IgE to Ara h 2 has been found to predict peanut allergy due to primary sensitization, whereas sensitization to Ara h 8 is more consistent with pollen-food allergy syndrome due to primary sensitization to Bet v 1. However, diagnostic cutoffs for Ara h 2 vary greatly between studies, and do not reliably predict reaction severity.²² CRDs are available for tree nuts such as hazelnut (Cor a 1, Cor a 8, Cor a 9, Cor a 14), walnut (Jug r 1, Jug r 3), cashew (Ana o 2, Ana o 3), Brazil nut (Ber e 1), and also for other foods such as sesame (Ses o 1), kiwi fruit (Act d 1, Act d 2, Act d 5, Act d 8), soy (Gly m 4, Gly m 5, Gly m 6) or shrimp (Pen m 1, Pen m 2, Pen m 4). Limited studies suggest better accuracy using CRDs for the respective tree nuts over whole extract; however, additional studies are needed.²² Some studies have suggested potential utility with predicting baked-milk and baked-egg outcomes using CRDs for milk allergens (Bos d 4, Bos d 5, Bos d 6, Bos d 8) and egg allergens (Gal d 1, Gal d 2, Gal d 3, Gal d 4, Gal d 5); however, data across studies are not consistent, nor do they demonstrate superior predictive ability over SPT and/or whole allergen specific IgE.¹³ Omega-5-gliadin (Tri a 19) is a wheat allergen associated with wheat dependent exercise induced anaphylaxis.^{33, 34} It has been correlated with IgE-mediated wheat allergy in Japanese and Swedish populations, but the usefulness in other populations is unclear.^{11, 22, 32} As with SPT, specific IgE testing (including CRD) has been shown not to predict severity with high accuracy and precision.¹¹

Specific IgE testing to allergen epitopes is an emerging diagnostic procedure that has been shown to identify patients with persistent cow's milk allergy³⁵ and has been shown to more accurately predict peanut allergy³⁶ and potentially eliciting dose thresholds.³⁷ Broad application to other allergens is currently limited, due to the need to validate the assays in large cohorts of well-characterized food-allergic patients.³⁸

2.6 Other tests

Basophil activation testing (BAT) is an in vitro assessment of allergen-induced basophil activation. Recent studies of BAT have reported some advantages of BAT over SPTs and specific IgE testing in peanut, milk and hazelnut allergy due to its higher specificity.^39-43 Limitations to broad implementation in clinical practice are the requirement for fresh blood (optimally, BAT should be performed within 4 h); impact of transportation on basophil reactivity (vibration and temperature changes may cause degranulation); patient-specific factors including non-responders (around 10% of individuals), concomitant inflammatory diseases, infection and others; and the need to identify large cohorts of well-characterized food-allergic patients to validate the assays.⁴⁴

Mast cell activation testing (MAT) is being investigated as a potential diagnostic tool for peanut allergy, although data are limited at the present time.^{45, 46} MAT uses plasma or serum to sensitize mast cells derived from either a cell line or primary cells from peripheral blood, tissue or engineered cell lines.^{38, 46} The MAT has demonstrated similar specificity in diagnosing peanut allergy as the BAT, but may be less sensitive, at least when using cell lines, for example, LAD2.³² MAT using primary, blood-derived mast cells showed improved sensitivity and specificity compared to BAT in one study.⁴⁷ Recently a Hoxb8 mast cell line, an immortalized progenitor line from the bone marrow of mice that are transgenic for the human high-affinity IgE receptor (FcεRIa) was established⁴⁶; preliminary studies suggest that this may also provide a more sensitive and reproducible assay for diagnosing allergy.⁴⁶

Among other tests, atopy patch testing has been investigated as a surrogate marker for determining clinical allergy in patients with atopic dermatitis, eosinophilic esophagitis and FPIES,⁴⁸ but its utility for diagnosis in food allergy is highly controversial and has been largely abandoned. The lack of standardized food extracts is a major limiting factor, and additional studies are needed to validate its use in diagnosis of food allergy.^{20, 49} Intradermal skin testing for foods is potentially dangerous, overly sensitive, increases the chance of a false-positive result and is not recommended.²⁰ Other modalities lacking sufficient evidence to recommend their use include allergen-specific IgG or IgG4 testing, pulse testing, electrodermal testing, lymphocyte activation testing, cytotoxic testing and applied kinesiology testing.⁴⁹

2.7 Conclusion

Oral food challenges remain the standard for diagnosing food allergy. The clinical history is the most important component to direct testing, and then a step-wise approach to confirmation typically includes skin prick testing and/or serum IgE testing with consideration for the use of CRD and/or epitope-specific IgE testing to support the diagnosis. The BAT is not currently widely available; however, there are efforts to bring BAT to clinical practice. MAT is an emerging diagnostic tool that may further assist with diagnosis in the future. Multivariate regression models have been developed and validated to predict peanut allergy and reaction severity outcomes from the DBPCFC with good diagnostic accuracy^{40, 50}; however, at this time, surrogate biomarkers have not replaced the need for a DBPCFC in the research setting.

3.1 Purpose of challenge

The indication for the challenge is important: whether it is being done for clinical reasons to prove or disprove a patient is allergic to a certain food, or for research purposes to confirm both clinical reactivity and establish the reaction threshold (eliciting dose) of food protein needed to cause a clinical reaction.^{56, 57} In addition, an OFC may be indicated for follow-up of food allergy when there is a significant chance that the patient has lost his allergy. The likelihood of a “reactive” challenge will depend upon the patient's history of previous reactions and clinical evidence suggesting possible resolution. The rate of reaction at OFC may vary according to the indication for the challenge^{58, 59}; of note, only around 50% or less of patients thought to have a food allergy will react, depending on the clinical setting.^{52, 60, 61} Other relevant factors include whether the suspected allergic reaction is likely IgE or non–IgE-mediated, the age of the patient and the food to be challenged.

3.2 Safety/challenge setting

There are several options for conducting OFCs: the intensive care unit, a regular hospital room, a hospital clinic room, an outpatient clinic room, an emergency room, a private practice office, or the patient's own home.^{62, 63} There is an inherent risk of severe reactions, including to very low doses. Around 10% of patients react with objective symptoms to doses under 10 mg food protein, and around 5% of these reactions will be anaphylaxis.⁶⁴ This holds true even in patients whose allergy may be perceived to be less significant. In one series of food challenges to raw milk/egg conducted in children tolerant to the allergen when present in baked foods, 18/87 (21%) developed dyspnea and/or mild hypotension,⁵⁸ some of them at milligram-level doses. Thus, the starting dose and schedule of dose escalation is part of the overall safety equation.

Certain foods seem to cause more severe reactions during OFC than others in different geographical areas, even when correcting for prevalence.¹¹ Peanut/tree nuts are the most common triggers in most regions; however, cow's milk (and other mammalian milks) and seafood are increasingly common causes of fatal and non-fatal anaphylaxis, and are even responsible for a higher rate of severe anaphylaxis than peanut in some regions.⁶⁵ In the European Anaphylaxis Registry, wheat has been reported to be responsible for more severe reactions compared to other foods in adults, possibly due to a greater influence of cofactors.^{66, 67} Surrogate tests, including allergen-specific IgE, CRD and BAT, do not correlate with the severity of symptoms provoked during OFC.¹¹ Some studies show a better association between sensitization to specific IgE components and likely severity, for instance sensitization to the 2S albumins in tree nut allergy.¹¹ The likelihood of a late-phase reaction or delayed symptoms should be considered prior to initiating an OFC, in order to determine the appropriate place and period of observation.^52-54 Other factors to consider include the age of the subject, the presence of other concomitant diseases such as atopic dermatitis, asthma, cardiovascular disease, or whether they have reacted to milligram amounts of the food by clinical history, the criteria used for determining a reactive challenge (objective and subjective symptoms), and whether the patient was on an elimination diet prior to challenge.^{1, 13, 53, 68-70}

An experienced physician will always have to make a safety assessment prior to any OFC, based on the various factors developed above. The setting will be chosen based on the pre-test risk assessment, keeping in mind that all OFC have an inherent risk of reaction.

3.3 Personnel

The personnel involved in a challenge procedure must be specifically trained in the management of acute allergic reactions, and anaphylaxis practice drills should be conducted periodically with the challenge team. Dedicated centers for doing food challenges are desirable, especially for challenges conducted for research purposes, so that personnel have adequate experience in conducting food challenges.⁷¹

3.4 Medications

All necessary medication should be readily available if needed during the food challenge. Epinephrine, oxygen, antihistamines, inhaled beta-agonists and intravenous fluids may all be needed to treat symptoms.^{1, 13, 71-73} Patients with a history of a prior severe anaphylaxis should have intravenous access established prior to initiating the challenge. The need for intravenous access should otherwise be guided by the ease by which access could be achieved in an emergency. The vast majority of individuals undergoing food challenge do not require intravenous access as a matter of routine. Patients with severe asthma, even without a history of prior anaphylaxis to food, and older children and adults with difficult intravenous access also should be considered for prior line insertion.⁵³

Patients can generally be discharged after an observation period of at least 1–2 h, provided no reaction occurred.¹³ If the clinical history indicates that allergic symptoms on the initial reaction occurred later, then a longer period of observation might be necessary. If the patient experienced significant clinical symptoms, then a period of up to 4 h of observation may be needed before discharge. If a patient experiences a severe systemic allergic reaction needing extensive treatment, then he/she should be kept under observation at the hospital overnight with appropriate treatment.^{52, 74}

3.5 Conclusions

Safety is an important consideration when deciding where and when a challenge should be performed. A food challenge may be done for various reasons, generally to identify a food that the patient should be avoiding or a food that the patient can add into their diet. It is important that OFCs be conducted in a location that is fully equipped to deal with anaphylaxis, by staff who are trained in dealing with such reactions. Appropriate settings also may include allergists' offices, since many allergists administering immunotherapy are fully equipped to manage anaphylaxis and are well trained in the management of OFCs.

4.1 General considerations

Prior to publication of the previous PRACTALL consensus document in 2012, numerous dosage schedules had been used for research DBPCFCs.^{29, 52-54, 57, 61, 75, 76} For example, Figure 1 shows several schedules for DBPCFCs with cow's milk. While schedules for other foods are similar, calculation from protein content to weight of food in either dried or native form may vary, especially when protein content of such foods are high (such as in fish) or low (such as in celery).⁵² As can be seen from Figure 1, starting doses vary widely from the low microgram level for studies aimed at determining the lowest observable adverse effect level (LOAEL) and no observable adverse effect level (NOAEL), to doses from the low milligram level to several hundred milligrams in schedules used for other purposes. Incremental scales vary from 10-fold increases through 5-fold,^{57, 77} semi-logarithmic (i.e., 1 – 3 – 10 – 30 – 100 – 300 – 1000 – 3000 mg etc.),^{29, 52} doubling doses⁷⁸ or even smaller increases,^{54, 61, 75} the latter being associated with schedules evaluating cumulative doses. Dosing intervals for various foods in relation to the aim of the challenge will be discussed in more detail later. In one study with peanut-allergic children undergoing OFCs in which the challenge was not discontinued on the emergence of symptoms except for anaphylaxis, 21 (78%) eventually developed anaphylaxis, but only three as the initial presenting symptoms. Thirteen (48%) children experienced initial non-anaphylactic symptoms, but then developed anaphylaxis with further peanut ingestion.⁷⁹ This implies that the incremental dosing used in OFCs can reduce the risk of anaphylaxis. However, in an analysis of 734 DBPCFCs in The Netherlands, dose predicted only 4.4% of the variance in reaction severity, that is, the relationship between dose and severity is complex and unclear.⁸⁰ This may be due to different reaction patterns in patients, with some demonstrating a dose–response but others not.⁸¹ Most datasets show that severe reactions can occur at all levels of allergen exposure, including milligram doses of allergenic food.¹¹

Due to differences in the reporting of safety and validity outcomes resulting from studies using different schedules, robust evidence demonstrating the relative superiority of any one schedule is limited. Comparing schedules is further hampered by the inability to assess the contribution of a given schedule parameter to a purported difference in outcome. Many parameters have been established on the basis of practical considerations, such as the length of time required to complete the challenge procedure.

Schedules developed for NOAEL/LOAEL determination employ starting doses at the low microgram level. However, it is more difficult to achieve meaningful top doses with acceptable increments in an acceptable period of time. A further consideration is the potential for partial desensitization (analogous to “rush desensitization”) and false negative results.⁸² A combination of logarithmic and semi-logarithmic increases, as done in CoFAR studies and EuroPrevall, can be helpful.⁸³

A distinction must be made between dosing schedules designed for different purposes: diagnosing a food allergy, performing an OFC before starting oral immunotherapy, determining reaction thresholds (for example, to counsel individuals on the degree of allergen-avoidance required), and conducting clinical trials. Several non-PRACTALL dosing regimens have been proposed in clinical settings. The dosing schedule typically includes between 3 and 6 steps, and a maximum dose (equivalent to an age-appropriate serving size) has been described for a selection of common foods (Table 3 and Table S2).¹³ There are scant recommendations in the literature concerning the number of challenge doses for open OFCs in a patient with a high likelihood of passing an OFC and without factors interfering with the development of an allergic reaction. In clinical practice, a similar total serving portion may be divided into fewer doses for a low-risk patient. Individuals who are sensitive to very low (milligram) levels of exposure can be assessed using a one dose challenge.⁵⁶ High starting doses may be associated with more severe reactions if utilized in an OFC.⁸⁴ Starting doses at the low milligram level are generally safe and seem to result in fewer severe reactions.^{85, 86}

The top dose required to avoid false non-reactive DBPCFCs is variable according to the protein content of the food being tested, but seems to be at least 2–3 g of food protein for most allergens. Sicherer et al.⁶¹ reported approximately 5% false negative test results with a schedule utilizing a maximum dose of 875 mg (cumulative dose 3500 mg) protein. Schedules with maximum doses of approximately 1750 mg (2190 mg cumulative dose) report a false negative rate of approximately 9% and 12.7%,⁸⁷ as gauged by recurrence of symptoms during subsequent introduction of the food. With the publication of population-based, dose-distributions for the majority of “priority” food allergens, there is now much more certainty as to the dose required to ensure a false negative test rate of under 5%. Suggested top-doses depend on both the proportion of protein in a particular allergenic food, but also need to acknowledge that some foods are less “potent” than others; for example, the amounts of protein to elicit an objective allergic reaction in 5% of people allergic to that food (ED05) are higher for white fish (12 mg, 95% CI 4.5, 43.9 mg) and even more so for prawn/shrimp (280 mg, 95% CI 69.3, 880 mg) compared to other priority allergens⁵⁶ as can be seen in Table 4. Top doses for these foods also need to be higher: published data on a safe schedule for fish and shrimp OFC have used a top dose of 12 g fish protein⁸⁸ and 9–16 g shrimp protein.^{88, 89} However, the top dose also needs to be adapted depending on the age of the patient, to account for age-related differences in serving size. For example, because of difficulties in getting infants to consume a 3 g protein amount, some studies have used smaller quantities (e.g., single top dose of approx. 2 g peanut protein⁹⁰ instead of 3 g).

Data suggest that up to 13% of OFCs may be false negative due to the challenge methodology using incremental doses that might induce transient desensitization in some patients, which seems to be independent of the allergen tested.⁸² At the same time, in other patients, the incremental exposure might “prime” the immune system resulting in less being tolerated during the OFC compared to a single exposure.⁹¹ Irrespective of the outcome, individuals who are non-reactive during an OFC should receive an age-appropriate serving size of the allergen in question following the challenge to confirm absence of reactivity. Some centers may choose to undertake this more formally, as a single-dose open food challenge performed on the next day or 2 h after the incremental challenge.

Data on the utility of a labial food challenge (LFC) as a starting dose are scarce. In one report, nine of 108 patients reacting to a LFC also reacted to subsequent OFC, while out of the 99 non-reactive LFCs, 27 OFCs turned out to be reactive.⁹² Another trial found that LFCs had poor sensitivity, and recommended against LFC as an alternative or initial step to formal OFC.⁹³ On this basis, including a labial dose in a challenge protocol is not recommended.

Japanese guidelines for food allergy include a stepwise OFC (on various days according to the reactions) for diagnosis and assessment of a food allergy with total cumulative loading dose, including low, medium, and full doses.⁹⁴ Stepwise OFCs can clarify the dosage of food that can be safely consumed^95-97 and allows to stratify individuals' long term prognosis.⁹⁸

Schedules will be different if the purpose is to observe for more delayed reactions, such as those seen in atopic dermatitis (AD), alpha-gal hypersensitivity, food protein-induced enterocolitis syndrome (FPIES), eosinophilic esophagitis (EoE) and food protein-induced allergic (eosinophilic) proctocolitis (FPIAP), as detailed below.

4.2 Other protocols

4.3 Food challenge schedules in the research setting

The semi-logarithmic dosing protocol results in a similar proportion of individuals allergic to the index allergen reacting to each dosing level (see Figure 2) and can be used to inform challenge doses for both research and non-research OFCs. Equivalent doses of priority food allergens are shown in Table S3, using a 7-dose regimen. For non-research challenges where there is less of a concern as to “first-dose” reactors (i.e., left-censored data), doses 1–3 can be combined resulting in a 5-dose regimen. For some foods, higher challenge doses are required, either due to low protein content or a lower potency of the allergen in provoking reactions (such that higher doses are needed); the latter is a particular concern with respect to shrimp and fish.⁸⁸ In addition, for some foods, the dose that would be expected to cause a reaction in the vast majority of individuals allergic to that food is significantly less than a typical serving portion (e.g., for cow's milk, soy milk, wheat-based products). Therefore, for these foods, an additional final “portion-size” dose is recommended (see Table S3).

4.4 Double-blind, placebo-controlled food challenges (DBPCFC) and open food challenges

The preferred method for DBPCFCs is the administration of active and placebo challenges on separate days. If the active and placebo test challenges are administered on the same day, the active and placebo challenge should be separated by at least 3 h. However, such schedules are not useful for diagnosing reactions that occur more than 3 h following the challenge. The working group does not recommend administration of active and placebo doses in an interspersed fashion, that is, administering both placebo and active doses in the same challenge, since reactions cannot then be attributed to a specific dose. If subjective symptoms occur and the observer is unsure whether a true reaction is developing, it is preferable to delay the next dose to see if symptoms progress.

Starting doses at the low milligram level are generally safe and seem to result in fewer severe reactions than higher doses.^{85, 86} For example, Hourihane et al.¹¹⁰ demonstrated that a dose of 1.5 mg peanut protein (equivalent to an ED05 exposure, that is, the amount of allergen that would be expected to trigger an objective reaction in at least 5% of the population allergic to that allergen) resulted in only mild symptoms in peanut-allergic children using a single dose-challenge. A starting dose of 3 mg peanut protein seems also to be safe as children with high levels of IgE-sensitization undergoing an OFC experienced only mild to moderate symptoms at this dose.^{118, 119}

Based on the above recommendations, a typical dosing schedule for a DBPCFC should include the following doses: 3, 10, 30, 100, 300, 1000 and 3000 mg of food protein (Table 3 and Table S3). Such a semi-logarithmic schedule is likely to be appropriate for most foods, patients, clinical situations and settings. In the clinical setting where open OFCs are undertaken in individuals who are not suspected to be low-dose reactors, the first three doses could be combined into a 43 mg protein dose, resulting in a 5-dose schedule. Sample schedules are provided for the priority food allergens in Table S3.

4.5 Dosing intervals

The previous PRACTALL document¹ recommended an interval of at least 15–20 min between doses. However, time from ingestion to reaction is longer than 15 min for most allergens and median time from ingestion to objective allergic reaction varies depending on the allergen: from 20 to 30 min for milk,^{120, 121} 55 min (range: 5–210 min) for peanut¹²² and 50–75 min for some forms of cooked egg.^{120, 121} These data support the hypothesis that shorter dose intervals may lead to an overlap effect and result in patients receiving higher doses, which could cause more severe reactions.¹⁰⁰ This may be a particular issue with food challenges where the allergen is in a non-native form, such as “baked-egg” and “baked-milk” challenges.¹²³ Ke et al.¹²⁴ studied variable dosing intervals of up to 30 min in milk-allergic children undergoing OFCs prior to immunotherapy and reported that longer dosing intervals were associated with less need for rescue treatment with multiple doses of epinephrine. Studies have reported less severe reactions using dosing intervals of 60 versus 30–40 min for milk,¹²⁵ egg,^{125, 126} boiled egg¹²⁵ and wheat.^{125, 127} On this basis, a dosing interval of at least 30 min is recommended for food challenges conducted in the research setting, and 20–30 min for clinical OFCs. Longer dosing intervals (e.g., 60 min) may be justified in some patients with a history of more severe reaction, or alternatively, clinicians might wish to extend the dosing interval where a patient is suspected to meet challenge stop criteria if the next dose is delayed (e.g., if 30 min after the dose, symptoms are evolving rather than resolving).

6.1 General considerations

To facilitate comparisons of DBPCFC outcomes, standards must be followed with respect to reporting results, including which symptoms are classified as subjective or objective, and what outcomes constitute a reactive challenge. It is not possible to have universally agreed upon published parameters, likely because clinical judgment is needed, and circumstances may vary by patient or study characteristics. Decisions to discontinue dosing may be made for reasons that vary according to requirements of specific study protocols, the symptoms observed, safety issues and patient characteristics. In response to safety concerns, dosing may be discontinued prior to eliciting clear objective reactions, but this in turn may reduce the diagnostic accuracy of the procedure.

Food challenges are typically considered reactive (positive), and dosing stopped, when objective symptoms are present. In some situations, however, mild objective symptoms may be considered insufficient to discontinue dosing or to consider a challenge reactive (e.g., one or two transient perioral urticarial lesions from contact with the food or one episode of vomiting in a child with anxiety and a distaste for the challenge substance).⁵¹ Subjective symptoms may in some circumstances indicate a reactive response to challenge and present a good reason to discontinue dosing, for example, by having repetitive symptoms or multiple subjective symptoms in several organ systems, Table 6. However, stopping a challenge with only subjective symptoms increases the risk of a false-positive test compared to discontinuing the challenge only for objective symptoms. Subjective or initially mild objective symptoms may be subtle in a young child, who may become suddenly quiet or have behaviors such as food refusal or scratching in the ears or at the tongue and neck.

At a minimum, the parameters for stopping and declaring a challenge reactive (positive) or negative should be pre-specified in challenge protocols and the details reported in publications. Symptoms typically considered subjective are discussed below. Regarding the options for discontinuing dosing and considering a challenge reactive, options include “worsening” subjective symptoms, repeated elicitation (e.g., on three doses) or persistence of symptoms (e.g., 2 dosing intervals, or 40–60 min).^51-53 These parameters for stopping a challenge and declaring a challenge reactive have not been evaluated with regard to impact on the sensitivity and specificity of the test. When mild objective or subjective symptoms occur, decisions include stopping the challenge, waiting longer for the next dose or repeating a dose.^{51, 70} Judgments about proceeding must balance safety against the certainty of the challenge outcome. Although more time consuming, undertaking additional challenges (e.g., 5 oral challenges per food with three challenges containing placebo and 2 challenges containing the food allergen or vice versa, randomly) can greatly increase the accuracy of conclusions when symptoms are subjective.⁵²

6.2 Stopping criteria for OFCs

Recent evidence supports the concept that there are significant differences in reaction thresholds when less objective symptoms are used to define the stopping point in food challenges, even with the same dosing schedule.¹³⁷ Less stringent criteria (i.e., modified CoFAR 3 versus prior PRACTALL guidelines) appear to underestimate the challenge reaction threshold.¹³⁸ This may be problematic in research studies where a precise eliciting dose/cumulative reactive dose (greatest tolerated dose/cumulative tolerated dose) define the primary endpoint, but not in clinical settings where the aim is to determine simply whether a patient is reactive (allergic) or tolerant to a specific food.

Typically, when food-allergic individuals react to their trigger allergen, there are two patterns of symptom evolution: the majority of individuals initially experience subjective symptoms (such as oral and/or pharyngeal pruritus or mild gastrointestinal discomfort) – although this can be difficult to communicate in very young children – prior to progressing to the onset of objective symptoms with or without further challenge doses being given. However, some individuals have a different pattern, where objective symptoms (which may include anaphylaxis) are the presenting symptoms (Figure 3).⁸¹ Ideally, OFCs (whether blinded or unblinded) should be stopped on the basis of reproducible and objective symptoms. Subjective symptoms (Table 6) are common during OFCs and often precede the emergence of objective symptoms by a number of dosing intervals.

Subjective symptoms, which can be substantial and persistent, are also reported during placebo OFCs,⁷⁷ which complicates the decision of when to discontinue an OFC and confirm the presence of food allergy. For example, if symptoms such as nausea or dyspnea (without objective signs) are a result of anxiety rather than true food allergy, this could lead to a false diagnosis of food allergy. Furthermore, there is considerable variability in how clinicians interpret subjective symptoms that occur during food challenge, and whether they indicate a reactive reaction.^{139, 140}

Mild nasal symptoms are common during food challenges in both food-allergic and non-allergic subjects¹⁴¹ and are therefore not usually considered indicative of true reactivity. Subjective oral symptoms, such as tongue and throat tingling or pruritus, present a particular challenge as they are difficult to distinguish from local oral symptoms. A retrospective chart review of 652 OFCs found that subjective oral symptoms occurred in over one third of food challenges.¹⁴² Subjective oral symptoms occurred with reactive OFCs, but the false-positive rate for subjective symptoms in predicting a reactive challenge was 30% – that is, among those who initially experienced subjective oral symptoms, 30% were able to complete the challenge without developing any objective symptoms. There was no difference in the occurrence rate of anaphylaxis during reactive challenges among those with and without subjective oral symptoms. In a separate study, which combined individual participant data from four interventional studies (n = 592), the use of less objective symptoms to define challenge stopping criteria underestimated both the challenge threshold and increased the rate of placebo reactions.¹³⁷ Together, these studies demonstrate the impact that subjective symptoms can have on the reliability of food challenge results, which is important in both research and clinical settings.

A clear approach is therefore needed to determine when to stop a diagnostic challenge and yet avoid the risk of false-positive tests. Furthermore, using subjective symptoms in isolation can significantly impact the dosing level reported to cause a reaction, which is typically used to inform primary outcomes in trials of therapeutic interventions in food allergy.¹³⁷ Therefore, an approach is needed to achieve a balance between safety (which may be impaired if excess, unnecessary doses are given during a challenge) and confirmation of true clinical reactivity.

A variety of approaches for handling subjective symptoms have been proposed, including waiting for the symptoms to resolve before continuing the challenge,¹⁴³ discontinuing the challenge if the subjective symptoms persist for a sufficient period of time, for example, 45–60 min,^{144, 145} or discontinuing the challenge if subjective symptoms occur over three consecutive doses.⁷ The previous PRACTALL guideline suggested that subjective symptoms that recur over three doses, or persist (e.g., 40 min) are more likely indicative of a reaction than when such symptoms are “transient and not reproducible”. However, this must be weighed against the observation that some subjective symptoms are less indicative of an allergic reaction than others (e.g., oral pruritus), and that a recent analysis of 592 reactive challenges from four separate studies found no significant adverse impact on rates of anaphylaxis during food challenges when excluding subjective symptoms from defining challenge stoppage.¹³⁷

Under the revised system, OFCs should be stopped if a single red symptom or two orange symptoms from different target systems are present during the same dosing interval. These criteria can be applied to both “open” (i.e., unblinded) and DBPCFCs. Staff performing challenges need to be suitably trained and supported to have confidence in implementing this guidance and not discontinuing challenges prematurely.

Peak flow can be assessed in most children over 6 years of age using a portable peak flow meter and can be a useful measure of bronchoconstriction (so long as technique is satisfactory). Where feasible, peak flow can be conducted at baseline and then as indicated during the challenge, to help assess whether subjective dyspnea is associated with objective change in respiratory function. Some groups have proposed using peak expiratory flow to help inform challenge stop criteria, on the basis that a fall in PEF of ≥20% from baseline is likely to indicate an objective impairment in ventilation.^{55, 146, 147} An assessment will need to be made as to whether any change in PEFR is due to poor technique/lack of effort or reflects bronchoconstriction. Modest reductions in peak flow can occur with abdominal pain by adversely impacting technique, but this is usually apparent to the observer.

When food challenges are used to evaluate reported delayed symptoms (e.g., isolated gastrointestinal reactions and atopic dermatitis) or more controversial atypical symptoms (migraine, behavior, and arthritis), different approaches, such as multiple DBPCFCs (with more than 1 active and 1 placebo challenge to assess reproducibility of symptoms), symptom diaries and SCORAD scores, might be needed. Similarly, evaluation of isolated oral symptoms from pollen-food allergy syndrome requires modification of outcome measures. Challenge protocols might require modifications to accommodate specific ages (e.g., smaller portion sizes/doses) or disease outcomes; nevertheless, it is essential to explicitly detail how symptoms are assessed with regard to stopping dosing and determination of reactive, negative, or inconclusive challenge results to allow comparison of outcomes. The 2012 iteration of PRACTALL guidelines contains a full discussion of right censoring statistical design and analysis issues for the DBPCFC when individuals exhibit subjective symptoms,¹ which can be found in Appendix S1.

7.1 OFC for research versus clinical settings, and reporting results

The DBPCFC is the “gold standard” for diagnosis used in research settings, and until a more reliable surrogate biomarker is developed, it is very likely that placebo-controlled challenges will be a standard part of research studies for the foreseeable future. In both research and clinical settings, the DBCPFC may be used to provide a definitive diagnosis of food allergy, to determine the dose-triggering threshold (eliciting dose) of an allergen, and to validate the acquisition of desensitization or tolerance to a full serving of the allergenic food.^{1, 94} The use of a placebo control allows for minimization of bias from both the participant and clinician administering the challenge; however, in clinical practice, placebo-controlled challenges are rarely used, especially in a double-blind manner.⁴ Single-blind challenges may be employed on occasion when performing a food challenge in a patient with a significant amount of anxiety that may prohibit their ability to reliably ingest a serving of the food without interference from subjective symptoms often provoked by anxiety. Open OFCs are the most common method of assessing tolerance development or validating the diagnosis of IgE-mediated food allergy in clinical practice. Open food challenges may be sufficiently reliable in infants who are unable to communicate subjective symptoms, or if stopping criteria requires objective evidence of an allergic reaction.

Reporting of protocols used and outcomes of DBPCFCs are key aspects to support reproducibility, assess for potential biases and to enable comparisons between studies. Where different stopping criteria are applied, this can preclude comparison of results based on food challenges. For example, the results of a trial that required ingestion of 300 mg of peanut protein without reaction to define desensitization compared to a trial that required ingestion of 1000 mg, are not directly comparable without acknowledging this key difference. The reasons for stopping a challenge should be reported, along with the number of participants in each category, in terms of objective symptoms only and then how subjective symptoms were handled (e.g., repeated subjective, worsening subjective, or persistent subjective symptoms). The number of patients with severe systemic reactions and their characteristics should also be reported. Caution should be applied in the use of epinephrine to treat a reaction as a measure of reaction severity, since multiple patient factors, site practices (e.g., the threshold to administer epinephrine might be lower in an outpatient setting), and physician judgment are often subjective, variable and will affect the treatment of a reaction. Nuanced aspects of the DBPCFC should be included in publications, irrespective of the study design (e.g., RCTs, observational studies etc.) including the type of matrix used for mixing the allergenic food, components of the placebo, including blinding ingredients, and the volume of the product required to ingest at each step of the food challenge. Table 8 includes a summary of data to report in studies using an OFC. Figure 4 illustrates the definitions of terms used in trial reporting. The PRACTALL working group recommends that trials report both the eliciting dose and cumulative tolerated dose in milligrams of protein prior to eliciting an objective, dose-limiting reaction. Investigators are encouraged to make individual data available when possible in on-line repositories.

Placebo responses should also be reported, including subjective and objective symptoms, and data should be reported in a similar fashion to the data from allergen challenges, including severity grading. The rate of placebo reactions (false positive rate) during OFC should be reported and accounted for in the design and analysis of the study.¹⁴⁸ Descriptions of symptoms elicited by an OFC may provide data for grading the severity of a reaction and should be reported. While severity grading has been extensively studied and discussed, it is not within the scope of the present guidelines.^{149, 150}

8.1 Key recommendations for conducting oral food challenges