Evaluation of the lower extremity veins in patients with suspected pulmonary embolism: A retrospective comparison of helical CT venography and sonography

OBJECTIVE, In patients undergoing a combined CT angiographic and CT venogra phic protocol, the accuracy of helical CT venography for the detection of d eep venous thrombosis was compared with that of lower extremity sonography. MATERIALS AND METHODS. Patients who had undergone a combined CT angiographi c and CT venographic protocol and sonography of the lower extremities withi n 1 week were identified. The final reports were evaluated for the presence or absence of deep venous thrombosis. Statistical measures for the identif ication of deep venous thrombosis with helical CT venography were calculate d. In each true-positive case, the location of the thrombus identified with both techniques was compared. All false-positive and false-negative cases were reviewed to identify the reasons for the discrepancies. RESULTS. Seventy-four patients were included. There were eight patients (11 %) with true-positive findings, 61 patients (82%) with true-negative findin gs, four patients (5%) with false-positive findings, and one patient (1%) w ith a false-negative finding. When comparing helical CT venography with son ography for the detection of lower extremity deep venous thrombosis, the se nsitivity measured 89%; specificity, 94%; positive predictive value, 67%: n egative predictive value, 98%; and accuracy, 93%. Of the eight true-positiv e cases, five had sites of thrombus that were in agreement on both CT venog raphy and sonography. Of the five discordant cases, four were false-positiv es and one was a false-negative. Possible explanations for all discrepancie s were identified. CONCLUSION. Compared with sonography, CT venography had a 93% accuracy in i dentifying deep venous thrombosis. However, the positive predictive value o f only 67% for CT venography suggests that sonography should be used to con firm the presence of isolated deep venous thrombosis before anticoagulation is initiated. In addition, interpretation of CT venography should be perfo rmed with knowledge of certain pitfalls.