CD02: Is it time to patch test methylisothiazolinone (MI) and methylchloroisothiazolinone/MI mix in petrolatum? A concerning recent rise in sensitization also revealed

Catherine Holden and Ruth Sabroe

Sheffield Teaching Hospitals NHS Foundation Trust, Sheffield, UK

We wanted to test the hypothesis that patch testing to methylchloroisothiazolinone (MCI)/methylisothiazolinone (MI) mix 0·02% in petrolatum (pet.) and MI 0·2% pet. is more sensitive than the same concentrations in aqueous (aq.). In 2021 these were added to an extended baseline series. Our results were startling. Regarding MI 0·2%, of 180 people tested in pet. 20·0% were positive (+/++/+++) vs. 11·4% of 236 tested in aq. Of 176 tested concurrently, 23% (n = 40) had positive reactions, of which 45% (n = 18) only in pet., 42·5% (n = 17) in both and 12·5% (n = 5) only in aq. Regarding MCI/MI 0·02%, of 127 people tested in pet. 12·6% were positive vs. 3·9% of 233 tested in aq. Of 120 tested concurrently, 13·3% (n = 16) had positive reactions, of which 62·5% (n = 10) only in pet., 31·2% (n = 5) in both and 6·3% (n = 1) only in aq., although the latter was positive to MI in aq. and pet., so would have been identified. Current relevance was found in 88·9% and 81·2% tested to MCI/MI in aq. and pet., 77·8% and 86·1% to MI in aq. and pet. In total, 120 people were tested concurrently to MCI/MI 0·02% aq. and pet., MI 0·2% aq. and pet., benzisothiazolinone (BIT) 0·1% pet. and octylisothiazolinone (OIT) 0·1% pet. Thirty-six (30%) were positive to one or more isothiazolinone, 28 (23·3%) to one or more MCI/MI or MI preparation. Fourteen (11·7%) were positive to one preparation only of which 35·7% (n = 5) to MI 0·2% pet., 7·1% (n = 1) to MI 0·2% aq. and 57·1% (n = 8) to BIT. One person was positive to all six haptens, this being the only person positive to OIT. Our data also show that there may be a new increase in sensitization to MI ± MCI. Rates of sensitization to MI tested in aq. were 18·0%, 6·0%, 8·1% and 11·4%, in 2015, 2018, 2020 and 2021, respectively, but rising to 20·0% in 2021 when tested in pet. We found a similar trend for MCI/MI in aq. (13·5%, 2·9%, 3·1% and 3·9% for the same years) rising to 12·6% in 2021 in pet. However, in 2020 and 2021 we tested only people with severe disease (due to COVID-19 restrictions), so the rates may be falsely high. Reassuringly, rates to BIT and OIT were relatively stable during this time-frame (BIT: 10·8%, 16·0% and 13·5% in 2015, 2019 and 2021, respectively; OIT: 1·7%, 2·9% and 0·9%, in 2015, 2019 and 2021, respectively). In summary, 62·5% and 45% of people positive to MCI/MI and MI, respectively, would have been missed if patch tested to these haptens in aqueous alone. We recommend testing in petrolatum. We also found a concerning rise in sensitization to MCI/MI and MI.