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A plasma inhibitor of ristocetin‐induced platelet aggregation in patients with sickle hemoglobinopathies

Because of the high prevalence of thrombotic complications in patients with sickle cell anemia (SCA), we investigated platelet function in patients with sickle hemoglobinopathies. Platelet aggregation induced by epinephrine, ADP, and collagen, except for absent secondary wave in 3 of 10 patients with SCA, was qualitatively normal. However, ristocetin‐induced platelet aggregation (RIPA) with a final concentration of 1.12 mg/ml was markedly abnormal – absent or virtually absent in 9 of 10 patients with SCA, 3 of 3 patients with hemoglobin S‐C disease, and 2 of 3 patients with sickle trait. All 8 controls used in these experiments repeatedly demonstrated normal RIPA. Addition of normal plasma failed to correct abnormal RIPA in sickle hemoglobinopathy patients. All patients demonstrated normal RIPA with a ristocetin dose of 2.24 mg/ml and aggregated with bovine fibrinogen. Recombinant mixing experiments demonstrated that washed SCA platelets support RIPA (1.12 mg/ml) when resuspended in normal plasma or high dilutions of SCA plasma, but not in undiluted SCA plasma. Washed normal platelets do not support RIPA (1.12 mg/ml) when resuspended in SCA plasma. These findings suggest the presence of a plasma inhibitor of RIPA in patients with sickle hemoglobinopathies.