Diagnosis of thrombosis evaluation of ¹²⁵I fibrinogen scanning and blood tests

At least 50% of patients who die of major pulmonary embolism have no clinical signs of venous thrombosis, yet the vast majority of these have evidence of thrombosis at autopsy. It is likely that early detection and adequate anticoagulant treatment of these silent thrombi would greatly reduce the mortality from pulmonary embolism. Of 150 patients who presented with a clinical diagnosis of venous thrombosis, 74 or 40% had normal venograms. Most of these patients had clinical signs of calf vein thrombosis but in some there was additional tenderness over the popliteal or femoral vein. Presumably, these patients had a local inflammatory lesion but did not have venous thrombosis. Of 100 venographically confirmed thrombi that were originally diagnosed by prospective ¹²⁵I fibrinogen leg scanning only 21 showed clinical evidence of venous thrombosis. The results of a comparison of expectant ¹²⁵I fibrinogen scanning with venography demonstrate an agreement between the two methods of better than 90% when both tests are performed in general surgical and medical patients. High levels of leg radioactivity in the absence of venous thrombosis are seen over hematomas, cellulitis and edema. High counts over hematomas limit the value of prospective scanning in patients undergoing hip surgery but even in such patients the agreement between scanning and venography is close to 90%. Prospective studies of high risk patients scanned expectantly were collected from the literature and showed a very low risk of pulmonary embolism in patients with a normal fibrinogen scan, and a 15 fold higher risk in patients in whom the scan was positive. Fibrinogen scanning is less reliable when it is used to detect established thrombosis because the test only becomes positive if fibrin is still being deposited onto the thrombus or if the thrombus extends. The authors found an agreement between diagnostic scanning and venography of 75%. In a predictive study (Gallus et al. 1973a) the protamine sulphate test and the ethanol gelation test were performed to detect circulating fibrin monomer complexes and the hemagglutination inhibition assay test of Merskey and colleagues (1972) to assay serum FDP preoperatively and daily postoperatively. A useful correlation between positivity of any of these tests and the development of ¹²⁵I fibrinogen scan thrombosis was not found. There was a 71% correlation between thrombosis detected by ¹²⁵I fibrinogen scanning and the presence of large mol wt fibrinogen complexes detected by gel chromatography using a modification of the method described by Fletcher and associates (1970). However, there was a relatively high incidence of false positive results which limited the diagnostic value of this test for the diagnosis of postoperative venous thrombosis. Although blood tests appear to be sensitive, they are unlikely to be specific for venous thrombosis since a considerable proportion of patients who are at risk of developing venous thrombosis also have conditions producing mild disseminated intravascular coagulation which could result in raised FDP levels, positive paracoagulation tests, and positive gel chromatography. Thus, these blood tests may be most useful when they are negative, allowing one to exclude a thromboembolic state. (More than 50 references).