THE PLATELET-SPECIFIC ALLOANTIGEN P1(A1) (HPA-1A) - A COMPARISON OF FLOW CYTOMETRIC IMMUNOPHENOTYPING AND GENOTYPING USING POLYMERASE CHAIN-REACTION AND RESTRICTION-FRAGMENT-LENGTH-POLYMORPHISM IN A SWEDISH BLOOD-DONOR POPULATION

Background: There is an increasing interest in the development of rapi d and reliable techniques for platelet alloantigen typing. Study Desig n and Methods: By use of standardized flow cytometry and a specific hu man alloantiserum, 236 Swedish blood donors were immunophenotyped for the platelet-specific alloantigen, Pl(A1) (HPA-1a). Results: Ten indiv iduals (4.2%) had low fluorescence intensities and were considered Pl( A1)-negative (HPA-1a-negative); all of them also demonstrated a Pl(A2) (HPA-1b/1b) genotype in a polymerase chain reaction and restriction f ragment length polymorphism (PCR-RFLP) assay of the underlying DNA pol ymorphism. The remaining population had clear positive fluorescence an d was regarded as Pl(A1)-positive (HPA-1a-positive). The fluorescence distribution histogram among Pl(A1)-positive (HPA-1a-positive) individ uals was dome-shaped, and those individuals who were homozygous for Pl (A1) (HPA-1a) could not be distinguished from those who were heterozyg ous. This finding was further substantiated by PCR-RFLP analysis of th e Pl(A1)/Pl(A2) (HPA-1a/1b) genotype; a heterozygous genotype was foun d among those having a medium fluorescence intensity as well as among those having a strong fluorescence intensity. Conclusion: Flow cytomet ry is a valuable tool for large-scale detection of Pl(A1) (HPA-1a). Ho wever, flow cytometry based on only one antiserum cannot distinguish b etween homozygous and heterozygous carriers of Pl(A1) (HPA-1a). For zy gosity testing and when platelets are difficult to obtain, the PCR-RFL P technique is the assay of choice.