SummaryAutoimmune thrombocytopenia can be diagnosed by demonstrating antiplatelet antibody in patient's serum using a platelet factor‐3 test. A procedure was developed to conduct the platelet factor‐3 test using a Fibrometer to record coagulation time. The procedure provides a more practical and consistent way of reading results than making visual observations. It is also simple enough for use as a laboratory method to evaluate thrombocyto‐penic patients. With this procedure, positive results were obtained for about one‐third of the thrombocytopenic dogs examined