Change in Platelet Factor 3 as a Means of Demonstrating Immune Reactions Involving Platelets: Its Use as a Test for Quinidine‐Induced Thrombocytopenia

An increase in platelet factor 3 activity of intact platelet‐rich plasma (PRP), measured as a shortening of the product I or Russell's viper venom time, has been found to accompany platelet damage. In the present study increased platelet factor 3 activity was found on incubation of PRP with hetero‐antibodies directed against platelets, as well as in incubation mixtures containing quinidine and the antibody of quinidine‐induced thrombocytopenia obtained from six patients. Sensitivity of this measurement as a test for quinidine purpura was found to be comparable to that of quantitative complement fixation and its use as an adjunct in the diagnosis of drug purpura is proposed.