Objective: The in vitro bleeding test (IVBT) is a sensitive and simple ex vivo method, simulating nonvascular primary hemostasis induced by injury of a small vessel. This review gives an overview of the factors influencing the test and describes the standard test after optimization at the Thrombostat 4000. The main intention is to summarize the various results obtained with this test and to indicate its possible application in the routine diagnostics. Of particular interest in this context is the development of the succeeding test system called the Platelet Function Analyzer (PFA-100). Data Sources and Selection Criteria: The literature since 1985 was analyzed reporting on the use of the IVBT for experimental and clinical reasons, and discussed by consideration of the own results and experiences. Results: After standardization and optimization and by consideration of the various variables influencing the method, the IVBT with the Thrombostat 4000 proved well-suited to detect very sensitively the inhibitory effect of aspirin and other drugs on platelet function. Therefore, it can be used for the control of thromboembolic prophylaxis with these drugs and the assessment of the perioperative bleeding risk of patients taking these drugs. Additionally, the IVBT can be a useful tool to document the perioperative and posttraumatic bleeding risk regarding disorders of the nonvascular primary hemostasis. This especially applies to hemodilution with particular respect to the influence of the infusion solutions administered. The IVBT proved to be clearly superior to the Simplate bleeding time (BT) for screening and control of therapy in von Willebrand’s disease and may replace the BT. The IVBT detects most of the congenital and acquired platelet disorders without being influenced by plasmatic coagulation disorders including those caused by therapeutic anticoagulation with heparin, heparinoids, hirudin, and coumarin. By the IVBT, the efficacy of drugs to improve the platelet function (e.g. desmopressin) can be controlled. There are several special in vitro applications of the test: detection of platelet-specific antibodies, quality control of platelet concentrates (PC), prediction of therapeutic efficacy of PC in non-immunized recipients. Finally, in a special modification (thrombocyto-penia-adapted IVBT) it can be even used to determine the bleeding risk and the efficacy of platelet transfusion in chronically thrombocytopenic patients ( < 50,000 platelets/μl). The disadvantage of the IVBT with the Thrombostat 4000 is that it shows still too much variation and needs too much labor and cost for general use as routine test. The succeeding method PFA-100 has considerably simplified the handling, although it has not solved all problems yet. The preliminary results document its superiority regarding the handling and the comparability of the results to those obtained with the Thrombostat 4000. Conclusions: The IVBT is a simple and sensitive tool to detect nonvascular disorders of primary hemostasis, which is well-suited for broad routine use if the new test system PFA-100 can be further developed.

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