Severe neonatal alloimmune thrombocytopenia and patients with HPA-1a-specific antibodies require transfusion of HPA-1a-negative platelets. Identifying HPA-1a-negative donors requires simple and reliable typing methods. Most existing techniques use polyclonal antibodies, are time consuming and involve platelet isolation. We have used a horseradish peroxidase (HRP)-conjugated recombinant IgG1 anti-HPA-1a (CAMTRAN007) to develop a rapid and reliable enzyme-linked immunosorbent assay (ELISA), which eliminates sample preparation and reduces the incubation and wash steps associated with traditional sandwich ELISAs. The assay uses simultaneous incubation of the monoclonal antibody RFGP56 to capture GPIIbIIIa from whole blood and the recombinant IgG1 antibody to detect captured HPA-1a antigen. It allows 96 samples to be typed in less than 1 h and can be used on stored samples. Initial testing of 85 samples of known HPA-1a genotype demonstrated that HPA-1a-negative samples had OD values of < 0.266, whereas HPA-1a-positive samples had OD values of > 0.6. Testing of 1862 random donor samples in two blood centres confirmed these OD cut-off values and identified 45 HPA-1a-negative samples (2.4%), all except one giving OD values of < 0.2. The remaining HPA-1a-negative sample had an OD value of 0.303. The HPA-1a status on all the negative samples and an equivalent number of randomly selected positive samples was confirmed by flow cytometry and polymerase chain reaction with sequence-specific primers (PCR- SSP).

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