Multicenter evaluation of the flow cytometry T-cell crossmatch - Results from the American Society of Histocompatibility and Immunogenetics-College of American Pathologists Proficiency Testing Program

Background. The performance characteristics and interlaboratory comparisons of the T-cell flow cytometry crossmatch remain largely unknown. Methods. This study was performed using data from the ASHI-CAP proficiency testing program. Four unknown sera and two unknown cells were sent to participating laboratories twice a year for 4 years. Results. In one survey in which different crossmatch techniques were compared, flow cytometry was slightly more sensitive than the antiglobulin method and considerably more sensitive than direct cytotoxicity. However, the proportion of participants in any given survey detecting antibodies in all sera expected to be positive was 50-60% and has not changed over the years. Failure to detect antibodies correlated with low antibody concentration, diluting the unknown serum by the testing laboratory, and with the instrument used. False positive results with normal sera were infrequent. Fluorescence intensity values were not standardized and were highly variable, but when fluorescence units reported by individual laboratories were divided by their own positive-negative cutoff values, results from different centers were more comparable, In general, fluorescence-to-cutoff ratios >5 correlated with complement binding activity, whereas values <5 denoted concentrations below those required to fix complement. Conclusions. Flow cytometry, as used by most centers, is highly sensitive and allows relative antibody quantitation. Furthermore, the data define objective parameters that may help to standardize the test and improve its predictive value in clinical transplantation.