Determinations of the platelet count, clot retraction, bleeding time, capillary fragility, and coagulation time were made in 64 normal subjects and in 404 patients suffering from various diseases.

The normal values for the platelet count done by an experienced technician using the Rees and Ecker method were: mean, 409,000 per cu. mm.; standard deviation, 68,000 per cu. mm.; normal range (M ± 2.σ), 273,000 per cu. mm. to 545,000 per cu. mm.

There was a statistically significant relationship between the platelet count and the results of tests of clot retraction, bleeding time, and capillary fragility, but there was no significant relationship between the platelet count and the coagulation time. Factors other than platelet count or platelet function which may influence the results of these tests are discussed.

The critical level of the platelet count, below which abnormal bleeding is likely to occur, was found to be approximately 190,000 per cu. mm. in primary thrombocytopenic purpura and 230,000 per cu. mm. in secondary thrombocytopenic purpura. However, platelet counts as low as 100,000 per cu. mm. were found in one patient without abnormal bleeding, and counts as high as 280,000 per cu. mm. were found in another patient with classical primary thrombocytopenic purpura.

In all patients in the active phase of bleeding in primary thrombocytopenic purpura and in most with secondary thrombocytopenic purpura, the bleeding time was markedly prolonged and clot retraction was definitely diminished. In approximately one half of the patients suffering from thrombocytopenia without bleeding or from thrombocytopenia complicating other hemorrhagic states, the results of these tests were abnormal. Capillary fragility was increased in approximately three fourths of the patients with primary thrombocytopenic purpura, one half with secondary thrombocytopenic purpura, and less than one half with thrombocytopenia without bleeding or with thrombocytopenia complicating other hemorrhagic states.

In the stage of recovery from thrombocytopenic purpura, dissociation of the results of the various tests was sometimes found. In some patients the platelet counts returned to normal hut abnormalities persisted in the tests of the bleeding time, clot retraction, or capillary fragility. In other patients the results of one or all of these tests returned to normal before the platelet count had reached the normal range. These results have been interpreted as evidence of variability in the functional capacity of the platelets.

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