The body maintains a biological balance between bleeding and clotting. It is able to maintain this balance when the plasma proteins ( blood clotting factors), which are involved, are qualitatively and quantitatively viable. A shift haemostasis balance may lead to a bleeding diathesis and this shift is usually as result of a reduced amount of a clotting factor. Intrinsic Pathway factor - use assay based on APTT (VIII, IX, XI and XII). Extrinsic Pathway and common pathway factors - use assay based on Prothrombin Time (II, V, VII and X) All coagulation factor bioassays test the ability of dilutions of a standard or test plasma to correct the clotting time of a plasma deficient in the specific factor to be assayed. There is a linear relationship between the clotting time and factor concentration on a log-log plot. A quantitative assay of an individual clotting factor is required to identify an inherited deficiency and to monitor replacement therapy. Occasionally an assay helps to distinguish acquired defects e.g. DIC, vitamin K deficiency, inhibitors.