Fibrinogen is the final essential building block of the clotting process. Thus, all of the preliminary "cause and effect" events in the clotting cascade rely on the work of this molecule to measure their success. The most commonly used laboratory method for measuring fibrinogen is the Clauss fibrinogen assay. The Clauss fibrinogen assay is a quantitative, clot-based, functional assay. The assay measures the ability of fibrinogen to form fibrin clot after being exposed to a high concentration of purified thrombin. Plasma samples are pre-diluted which minimize assay interference from substances like heparin and fibrinogen degradation products. In brief, the diluted plasma is incubated at 37°C prior to the addition of the pre-warmed (37°C) thrombin reagent. From the exact moment of the addition of thrombin, the time to clot is measured. The clotting time in seconds is interpolated from a standard curve made using various dilutions of assayed standard plasma. The following chapter includes detailed information on the Clauss fibrinogen assay. Other fibrinogen assays used include fibrinogen levels derived from prothrombin time assays and antigenic methods. Fibrinogen measurements using the prothrombin time and antigenic based assays are described in brief.