Rapid screening of platelet donors for Pl^A1 (HPA-1a) alloantigen using a solid-phase microplate immunoassay

Pl^A1 and Pl^A2 are alternative platelet-specific alloantigens in the Pl^A1 system. Sensitization to Pl^A1 underlies most cases of neonatal alloim- mune 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 Pl^A1-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^® kit (Immucor, Norcross, GA). Platelet monolayers in microtiter wells were incubated with anti-Pl^A1, 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-Pl^A1 to Pl^A1-positive platelets whereas sedimentation of unattached RBCs into a central pellet indicated the platelets were Pl^A1-negative. Of 520 donors, 15 (2.88%) tested Pl^A1-negative, which correlates well with the reported Pl^A2,2 frequency in whites of 2.25 percent. Results were confirmed by DNA genotyping and/or immunoblotting. This screening technique permits phenotyping donors for Pl^A1 alloantigen with minimal specialized equipment. Confirmatory testing for Pl^A2 alloantigen can be reserved for donors that test negative for Pl^A1.