Hemostasis is a balance between complex interactions of directly opposing systems (coagulation and fibrinolysis) with seemingly unrelated systems at both the enzymatic and cellular levels (platelets, endothelium, leukocytes). It should not be surprising that coagulation disorders often accompany many different disease states. Because the function of each protein involved in coagulation is now better defined, newer methodologies have been developed to assay them. In this regard, the hemostasis laboratory can take a two-step approach to diagnosis: global screening and targeted analysis. Several new global test systems provide more detailed, quantitative, and more physiologically relevant evaluations than earlier assays allowed. In addition, individual enzymes, inhibitors, cellular release products, and low molecular weight products of activation reactions (molecular markers) can now be measured in sensitive, specific assays. With this new perspective, genetic predisposition to pathologic hemostatic conditions can be identified through molecular biology and can be identified during the early stages of disease (i.e., at subclinical stages before major pathologic complications are established), and more specifically targeted prophylactic, as well as therapeutic drug interventions, can be administered. The molecular markers of hemostatic activation that can be assessed by various immunochemical methods provide very early evidence of thrombotic, fibrinolytic, or platelet-involved aberrations. Technological advances in methodology and instrumentation have changed the scope of all clinical laboratories but, in particular, that of the coagulation laboratory. The dramatic growth and development have resulted from the influences of clinical chemistry, clinical immunology, pharmacology, biochemistry, and biotechnology. Analytical instruments for use in hemostatic testing go beyond plasma or whole blood clot-based readers, platelet aggregometers, and microscopes to a range of automated, discrete, chemistrylike analyzers, spectrophotometers, microliter ELISA systems, RIA systems, and multiprobe instruments designed to measure simultaneously the different assay end-points of colorimetric and clotting assays and flow cytometers. Instruments are designed for batch processing of single tests on multiple samples or multiple test panels on a single sample. Versatility ranges from instruments that measure only the final reaction solution, by end-point or kinetic analysis, to instruments that automatically pipet reagents and sample, incubate, and analyze the reaction for truly walk-away assay performance. Typically, a wider range of assays are available in automated laboratories as opposed to laboratories performing manual assays.(ABSTRACT TRUNCATED AT 400 WORDS)