Assessment of Inter-Laboratory Variability for Flow Cytometric Crossmatch Testing: Lessons Learned from Proficiency Surveys

Detection of antibody directed against human leukocyte antigens (HLA) using a combination of flow cytometric crossmatch (FCXM) and antibody tests, is an important responsibility of Histocompatibility laboratories. Proficiency testing surveys utilize the results of these assays to assess concordance across multiple laboratories. In this study, we reviewed the ASHI Proficiency Testing (PT) antibody and crossmatching (AC) survey results obtained over a 6-year period, to evaluate the degree and nature of inter-laboratory FCXM and antibody assay variability. National and international laboratories representing 22 countries produced >10,000 T cell and >10,000 B cell FCXM results. Based on the 80% consensus threshold established for FCXM surveys, 92.5% T cell FCXM and 91.7% B cell FCXM results reached positive or negative consensus and were respectively consistent with the presence or absence of donor specific HLA antibodies (DSA) that reached a 90% consensus. The 7.5% of T cell and 8.3% of B cell FCXM results that did not reach consensus were associated with a combination of consensus and non-consensus DSA. This analysis shows that despite differences in testing protocols and algorithms, there is good consensus for the FCXM assay among laboratories. The data show correlation between FCXM and bead-based assays and support the use of both for reliable information when assessing immunological risk.