The process of fibrinolysis involves the enzymatic cascade which helps to break down cross-linked fibrin molecules. The most relevant factor is tissue type plasminogen activator, which converts plasminogen to plasmin which cleaves fibrin into various cleavage products. Tests for measuring overall fibrinolytic activity, such as the previously utilized fibrin clot lysis methods, are of no value in critical care management. Assays exist which quantitate breakdown products of fibrin (fibrin degradation products) and fibrinogen, according to the principle described for clotting cleavage fragments. More specific and very sensitive assays are now being developed, mostly aimed at the D-dimer fragment of fibrin, and these promise to be a valuable addition to the laboratory armory. Very simple bedside applications of the D-dimer test are currently being evaluated in thromboembolic disorders. Again, the main drawback is the high sensitivity and limited specificity; it appears that in many disorders D-dimer plasma levels are elevated, such that these assays are of no value in establishing the diagnosis of thromboembolism or disseminated intravascular coagulation (DIC). However, D-dimer assays may be useful for excluding the presence of thromboembolism, provided that comorbidity that may cause coagulation and fibrinolysis derangements is virtually absent. Unfortunately, this will not be the case in critical care patients.

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