Thrombocytopenia-adapted in vitro bleeding test assesses platelet function in thrombocytopenic patients. 
Platelet counts do not always reflect the true bleeding risk in chronically thrombocytopenic patients, and the posttransfusion platelet increments do not necessarily demonstrate that therapeutic efficacy.  There are no easy and reliable tests yet permitting the determination of platelet function in thrombocytopenic patients.  The in-vitro bleeding test (IVBT) with the Thrombostat 4000 proved to be a very sensitive and specific test for the detection of platelet disorders.  In order to become suitable for the investigation of thrombocytopenic blood with platelet count between 5 x 10(9)/L and 50 x 10(9)/L, special modifications were necessary.  We report on the evaluation of two thrombocytopenia-adapted modifications (TP-IVBT 150/120), first with blood of healthy donors made thrombocytopenic (three experiments with six blood samples each of different platelet concentrations and identical hematocrit) and then in a clinical study on 77 thrombocytopenic patients (69 with bone marrow hypoplasia, eight with autoimmune thrombocytopenia) receiving 267 platelet transfusions.  The patients were followed over 15 days on average (1-67 days) by daily examinations (total 1,285 observation days).  Most TP-IVBT measurements were carried out in triplicate, using the modification with the 120-microns filter (TP-IVBT 120) because it proved to be superior to the other modification.  Additionally, cell counts, hematocrit, body temperature, platelet volume, platelet distribution width, expression of CD 36, 41a, 42b on platelets, Simplate bleeding time, and detailed analysis of bleeding signs were performed for the calculation of a bleeding score.  There was a close correlation between TP-IVBT and platelet counts with thrombocytopenic normal blood (r2 = 0.81-0.94).  This indicated the suitability of this test modification to examine platelet function in thrombocytopenic patients.  The clinical study showed that the TP-IVBT helped at least to determine the platelet-related bleeding risk in thrombocytopenic patients.  It allowed differentiation between hypoplastic and autoimmune thrombocytopenia in most cases.  In addition, significant differences in platelet function of various diseases and of different bone marrow regeneration could be demonstrated.  The TP-IVBT is well-suited for the control of platelet transfusion efficacy and may replace the in-vivo bleeding time in most cases.  On the other hand, the test still shows too much of a variation and involves too much labor and cost for routine application.