A method to determine the amount of circulating platelets without the aid of any special apparatus is described. The method consists in separating the platelets from the other blood elements by fractional centrifugation and in measuring the light absorption of the platelet suspension in a photoelectric apparatus. Light absorption is standardized against the platelet volume found by the Van Allen thrombocytocrit, and results presented as platelet volume in 100 ml. of bood.

The error of the method was found to be 6 per cent. To test the consistency of the method, normal dogs and patients of both sexes with different pathologic conditions were studied, the results agreeing with the data found in the literature. In dogs of both sexes, twenty males showed 0.50 ml. per 100 ml. blood with a standard deviation of ±0.15, and ten females 0.53 ml./100 ml. ±0.17; fiftytwo male patients showed 0.45 ml. per 100 ml. of blood with a standard deviation of ±0.20, and thirty-one females 0.44 ml./100 ml. ±0.23.

Still further to test the reliability of the method, a confirmation was undertaken of the fact of platelet increase in the postoperative period in major surgical interventions. Twenty-seven patients with 0.38 ml./100 ml. (±0.16 as standard deviation) platelets showed soon after the operation 0.44 ml./100 ml. ±0.19 a difference not significant according to statistical tests; eleven days after the operation, average platelets were 0.60 ± 0.21 ml./100 ml. a highly significant increase.

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