Diagnosis of EDTA-dependent pseudothrombocytopenia in a horse.

Thrombocytopenia in horses may be idiopathic or secondary to chronic infectious or inflammatory diseases (eg, equine infectious anemia, lymphosarcoma), drug administration, bone marrow depression, myelophthisic disease, or disseminated intravascular coagulation. This report describes EDTA-dependent pseudothrombocytopenia in a horse. Platelet counts for blood containing EDTA were consistently less than reference range, but platelet counts of blood containing heparin were within reference range. When thrombocytopenia is diagnosed in horses without clinical evidence of a bleeding tendency, EDTA-dependent pseudothrombocytopenia should be considered. The diagnosis can be confirmed simply by screening blood films for platelet clumps and by comparing platelet counts of paired blood samples, one containing EDTA and the other containing heparin.