Immunohistochemical validation of rare tissues and antigens with low frequency of occurrence: Recommendations from the Anatomic Pathology Patient Interest Association (APPIA)

Laboratories worldwide find it challenging to identify enough tissues and cases for verification and validation studies of low-incidence, rare antigens. These antigens have a low frequency of occurrence in the population, or have little or no expression in normal tissues. Validation studies are essential to assure testing standardization before introducing a new instrument, product, or test into the clinical laboratory. The College of American Pathologists has published comprehensive guidelines for the verification and validation of new immunohistochemical tests introduced into the laboratory menu. Within the guidelines, varied numbers of cases are required for nonpredictive versus predictive markers. However, regarding low-incidence antigens, the laboratory medical director determines the extent of validation required. Recommended practical solutions available to clinical laboratories for low-incidence validation include developing internal resources using the laboratory information system with retrospective and prospective search(s) of archival material and purchase of tissue microarray blocks, slides, or cell lines from external resources. Utilization of homemade multitissue blocks has proved to be extremely valuable in biomarker research and demonstrated great utility in clinical immunohistochemistry laboratories. Participation in External Quality Assessment program(s) may provide insufficient numbers or the ability to calculate concordance rates. However, supplementation with in-house tissues can allow a laboratory to reach the optimal number of cases needed for verification and/or validation schemes. An alternative approach is conducting a thorough literature search and correlating staining patterns of the new test to the expected results. These solutions may be used uniquely or together to assure consistent standardized testing.