Quantitative measurement of fibrinogen is most commonly done using the Clauss technique, in which the clotting time of dilute plasma after the addition of thrombin is measured. At high thrombin concentrations (>30 NIH units/mL) and low fibrinogen concentrations, the fibrinogen level is inversely proportional to the thrombin induced clotting time.
Bovine and human thrombins have been used successfully in Clauss assays. FibroTek uses purified human thrombin.
The FibroTek Fibrinogen test is performed by adding 1 volume of Thrombin reagent to 2 volumes of diluted patient plasma and recording the time to clot formation.