Studies are reported on a patient who developed thrombocytopenic purpura following quinidine therapy. Quinidine caused lysis of the patient’s platelets in vitro within 40 minutes and complete inhibition of clot retraction. Platelet lysis preceded platelet agglutination. Increasing the concentration of the sodium citrate used as anticoagulant inhibited the in vitro action of the quinidine. No thrombocytolytic effect of the patient’s serum with quinidine could be shown when tested against normal blood.

A test dose of quinidine administered orally after the patient had recovered from the thrombocytopenia produced a profound decrease in the number of platelets within 90 minutes, associated with a prolongation of the bleeding time and disappearance of clot retraction.

The in vitro hypersensitivity to quinidine persisted for at least four weeks after recovery from the purpura; after six weeks it could no longer be demonstrated.

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