A test for platelet thromboplastic function measuring the activation of platelet factor 3 on incubation of ADP with citrated platelet-rich plasma has been standardized in terms of ADP and platelet concentrations, and amount of agitation, to give sensitive and reproducible results. Abnormal results have been found in patients with elevated blood urea nitrogen; the abnormality is due to a plasma inhibitor and is reversible on the addition of small amounts of calcium. Guanidinosuccinic acid, a newly described metabolite found in the urine of patients with azotemia, also inhibits ADP-induced platelet factor 3 activation. The properties of guanidinosuccinic acid and the uremic inhibitor so far studied suggest that they may be identical.

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