The sensitivity and specificity of 2 antibody tests for diagnosis of idiopathic thrombocytopenic purpura (ITP) in dogs were investigated prospectively. An ELISA to detect antibodies bound to the surface of platelets from affected dogs (direct test) was performed in 34 dogs with a clinical diagnosis of ITP and in 21 dogs with thrombocytopenia attributable to other causes. An ELISA to detect platelet-bindable antibodies in serum from affected dogs (indirect test) was performed in 32 dogs with ITP and in 15 dogs with other causes of thrombocytopenia. The direct test was positive in 32 of 34 dogs with ITP (sensitivity, 94%) and negative in 13 of 21 dogs with other causes of thrombocytopenia (specificity, 62%). Positive direct test results were obtained in 2 dogs with systemic lupus erythematosus, and in 1 dog each with concurrent Ehrlichia canis and Babesia canis infections, dirofilariasis, myelodysplasia, disseminated intravascular coagulation (of unknown cause), and thrombocytopenia subsequent to administration of trimethoprim/sulfadiazine, as well as in 1 dog with thrombocytopenia 14 days after a whole blood transfusion. The indirect test had positive results in 11 of 32 dogs with ITP (sensitivity, 34%) and negative results in 12 of 15 dogs with other causes of thrombocytopenia (specificity, 80%). Positive indirect test results were obtained in 1 dog each with systemic lupus erythematosus, concurrent E canis and B canis infections, and thrombocytopenia subsequent to administration of trimethoprim/sulfadiazine. Detection of platelet-bound antibodies was more sensitive than detection of serum-platelet bindable antibodies in confirming a diagnosis of ITP in dogs.(ABSTRACT TRUNCATED AT 250 WORDS)