When necessary, the final study design for assessing the reference interval is up to the laboratory director-s discretion. Accredited laboratories and laboratories operating in exempt states should contact their appropriate agencies to confirm any additional requirements for assessing the reference interval. Additional information on verifying or establishing assay reference intervals can be found in the Clinical and Laboratory Standards Institute (CLSI) guideline EP28-A3c and the accompanying Implementation Guide. The reference interval characteristic is not applicable for microbial identification systems, and therefore does not need to be assessed as part of a verification or validation study. No specific recommendations are provided for assessing reference intervals in organisms with intrinsic resistance to certain antimicrobials; the study design is therefore up to the laboratory director-s discretion. For multiplex molecular assays where negative/not detected is the reference interval, it is not necessary for laboratory personnel to verify or validate this characteristic.