Two-Stage Clotting Factor Assay

**Semantic type:** Laboratory Procedure

**Definition:** A two-staged test with purified coagulation factors in the first stage, and a FXa-specific chromogenic substrate in the second stage. In the first stage, diluted sample plasma, which has been treated with Al(OH)3 to remove prothrombin, is mixed with a combined reagent (human serum, factor V, phospholipids and calcium). In the second stage, a subsample from the first stage is mixed with normal plasma, providing prothrombin and fibrinogen, and the clotting time is measured to deduce the amount of FVIII present.

**Synonyms:** - Two-Stage Clotting Assay