Quantitative laboratory tests with high accuracy and precision are mandatory for the correct diagnosis of coagulation disorders. A blood specimen must be collected aseptically using a disposable plastic syringe, avoiding hemolysis and contamination from the tissue juice. For a special clotting test, the two-syringe technique is recommended.The internationally accepted procedures (ICSH, ICTH, etc.) recommend an anticoagulant concentration of 3.2% tri-sodium citrate·2H2O (0.109M). Commercially available reagents for blood coagulation tests are of the two types: activated partial thromboplastin time (APTT) reagent and prothrombin time (PT) reagent. They differ in the source organs from which they are extracted and the purification procedures by which they are prepared. Also, various automated instruments based on several mechanism such as electrical, optical, and mechanical, are commonly used. Although low CVs (coefficient of variation) are obtained when APTT or PT is determined by the use of an automated instrument, the quantitative parameter, such as clotting time and the shape of standard curve, critically depends on the type of reagent or instrument, as well as on the combination of them. Identical end point is not always realized when another reagent or instrument is used for the assay. In this respect standardization of the assay procedures for universal acceptance is storongly desired.In an immunological method an optimal combination of antigen and antibody concentrations is a prerequisite for a reliable antigen-antibody reaction. In a reaction with a synthetic chromogenic substrate the most crucial factor is the precision in the reaction time. These assays are specific only when cross-reactions of these reagents with other factors present in the specimen is negligible. Because these assays are based on one of the following activities, clotting, immunological, and amidolytic, which may or may not correlated very well with each other, through understanding of the assay specificity is essential for the correct diagnosis.