When blood clotting occurs in the body, thrombin acts on fibrin and turns it into cross-linked fibrin. At the same time, the fibrinolytic system is activated, and the various fragments, dimers, multimers and complexes produced by the degradation of fibrinogen and fibrin are collectively called fibrin degradation products (FDP). The FDP assay measures amounts of the fibrin and fibrinogen split products in the blood and directly indicates the level of activity of the fibrinolytic (clot-busting) system.