Rapid screening of platelet donors for PlA1 (HPA-1a) alloantigen using a solid-phase microplate immunoassay

PlA1 and PlA2 are alternative platelet-specific alloantigens in the PlA1 system. Sensitization to PlA1 underlies most cases of neonatal alloimmune thrombocytopenia (NAIT) and posttransfusion purpura (PTP) in white populations. A rapid and simple method for large-scale platelet phenotyping is desirable for identifying expectant mothers at risk of allosensitization and for identifying PlA1-negative donors when transfusions are indicated for treatment of NAIT or PTP. We investigated the effectiveness of a solid-phase microplate immunoassay for this purpose. Platelet-rich donor plasmas were tested using the Capture-P(R) kit (Immucor, Norcross, GA). Platelet monolayers in microtiter wells were incubated with anti-PlA1, washed, and exposed to red blood cells (RBCs) precoated with anti-human IgG. Adherence of RBCs in a diffuse pattern across the well surface indicated the attachment of anti-PlA1 to PlA1-positive platelets whereas sedimentation of unattached RBCs into a central pellet indicated the platelets were PlA1-negative. Of 520 donors, 15 (2.88%) tested PlA1-negative, which correlates well with the reported PlA2,2 frequency in whites of 2.25 percent. Results were confirmed by DNA genotyping and/or immunoblotting. This screening technique permits phenotyping donors for PlA1 alloantigen with minimal specialized equipment. Confirmatory testing for PlA2 alloantigen can be reserved for donors that test negative for PlA1.