144-P: COLLEGE OF AMERICAN PATHOLOGISTS HLA-B 27 PROFICIENCY TESTING DATA REVEALS INCREASED FALSE POSITIVE RATES FOR FLOW CYTOMETRY METHODS

Abstract

Aim Testing for HLA-B27 for disease association is performed using a variety of methodologies. Many laboratories use flow cytometry, a method typically not used for other HLA typing purposes. We evaluated five years of College of American Pathologists (CAP) HLA-B27 proficiency test (PT) results in order to compare flow cytometry to other methods. Methods We examined the results of the HLA-B27 CAP PT survey for 2008–2012. Results were classified as flow cytometry, molecular methods and microcytotoxicity. Results A total of 50 samples, 24 negative and 26 positive, were distributed. 6,130 of 6,370 (96%) results were classified as flow cytometry (3,870; 60.7%), molecular methods (1,617; 25.4%) or microcytotoxicity (643; 10.1%). The remaining results were obtained using more than one method in combination. The number of false negative results reported for the samples that were positive for HLA-B27 by method was 19/1991 (0.95%) for flow cytometry, 11/848 (1.30%) for molecular methods and 6/336 (1.79%) for microcytotoxicity. The number of false positive results reported for the samples that were negative for HLA-B27 by method was 41/1879 (2.18%) for flow cytometry, 1/769 (0.13%) for molecular methods and 5/307 (1.63 %) for microcytotoxicity. The false positive results from the negative samples were further analyzed for the HLA-B7 antigen in the sample. Eight of the 24 HLA-B27 negative samples were positive for the HLA-B7 antigen. Although these samples represented only 33 percent of the HLA-B27 negative samples, they accounted for 35 of the 41 (85%) incorrect responses by flow cytometry, while only accounting for 2 of the 5 (40%) incorrect responses by microcytotoxicity and none of the incorrect responses by molecular methods. Conclusions HLA-B27 typing by flow cytometry is more likely to yield false positive results in the presence of HLA-B7 than is typing by molecular methods or microcytotoxicity. The rate of false negative results when HLA-B27 was present was similar amongst the three methodologies.