We increased the specificity of flow cytometric detection of platelet-associated immunoglobulin (PAIg) by a combination of platelet gating and cutoff for positivity determined by the use of receiver operating characteristic (ROC) curve analysis, and we evaluated the significance of elevated PAIg in non-immune thrombocytopenic purpura (ITP) patients. Blood samples from 118 patients with a platelet count <100 x 109/L were used in this study. Flow cytometric detection of PAIg was performed. To obtain the cutoff of the surface-bound immunoglobulin for the discrimination of ITP and non-ITP, ROC curve analysis was used. The sensitivity of a positive PAIgG and PAIgM test for ITP in thrombocytopenic patients was 74.6%; the specificity was 79.7%; the positive predictive value 78.6%; and the negative predictive value 75.8%. Among 3 patients with myelodysplastic syndrome, 2 showed increased PAIg. Six of 20 patients with benign disease showed positivity for PAIg. Among these patients, 4 with elevated PAIg were diagnosed with liver disease. This study demonstrates that flow cytometric detection of PAIg combined with ROC curve analysis is a convenient, sensitive, and specific test, compared to previous methods, and it is useful for the differential diagnosis of thrombocytopenic patients.