BACKGROUND: Analysis of reticulated platelets (RPs) is useful for discriminating the causes of thrombocytopenia and monitoring the thrombopoiesis. In the patients with severe thrombocytopenia, we evaluated the thrombopoiesis-discriminating ability of several indices applying forward scatter (FSC) and thiazole orange (TO) fluorescence in addition to the percentage of reticulated platelets (RPs%). METHODS: Forty cases with decreased thrombopoiesis, twenty cases with increased thrombopoiesis and twenty cases with liver cirrhosis were selected. By flow cytometry with two analytic methods, dependent on or independent of the staining of CD41-PE as a platelet marker, the primary parameters including RPs% were measured and the applied parameters were calculated from them. And we compared the diagnostic efficiency of each parameter and analyzed the purity of platelet light scatter gate. RESULTS: The purity of platelet light scatter gate was significantly lower in patients with severe thrombocytopenia than in healthy persons with normal platelet counts (P<10(-6)), so the use of CD41-PE for platelet gating improved the diagnostic efficiency of RPs%. Compared to the primary parameters, the applied parameters originated from RPs%, FSC and TO fluorescence improved diagnostic efficiency significantly (RPs%: 55%, RPs%xs delta MFI: 80%) between decreased and increased thrombopoiesis groups. CONCLUSIONS: In the patients with severe thrombocytopenia, the estimate of the thrombopoiesis by a flow cytometric analysis can be more predictable by using platelet markers and by considering the fluorescence intensity of TO together with the RPs%.