Immune thrombocytopenias: tests for platelet antibodies

In a study of 49 consecutive patients for whom a test for platelet antibodies had been ordered, tests were performed for both cell-bound and serum platelet antibodies. In cases of autoimmune thrombocytopenia (ITP), the test result for cell-bound antibody was more likely to be positive than was the test result for serum antibody. In three patients with ITP who were taking systemic corticosteroids, however, the result of the CBPAT was negative. A similar phenomenon occurred in a patient with SLE. The result of the SPAT correlated best with the presence of alloantibodies formed during pregnancy or after blood transfusion.