Laboratory parameters to monitor safety and efficacy during thrombolytic therapy. Thrombolytic therapy is being used with increasing frequency in myocardial infarction (MI), pulmonary embolism, deep venous thrombosis (DVT), and peripheral arterial occlusion. Use of these agents, however, is hampered by concerns regarding safety and efficacy. Numerous laboratory parameters have been evaluated for monitoring the risk of bleeding complications, with levels of fibrinogen (and percentage of decrease) and fibrin/fibrinogen degradation products (FDPs) correlating to a variable extent with clinical bleeding. The bleeding time (BT) test has also been proposed as a potential predictor of bleeding during thrombolytic therapy. With respect to efficacy, the D-dimer fragment of FDPs, when corrected for soluble fibrin polymers, has been shown to correlate with clot lysis in venous thromboembolism but not MI. The BT also is being considered as a marker of lysis in patients with DVT. Given the increasing concomitant use of antiplatelet agents during thrombolytic therapy, the BT, as an in vivo test of hemostasis and platelet function, has potential utility as a noninvasive, adjunctive marker of thrombolytic efficacy. 