How accurate are ventilation-perfusion scans for pulmonary embolism?

To evaluate the accuracy of ventilation-perfusion scanning in the diagnosis of pulmonary embolism, the pulmonary arteriograms of 55 patients suspected of pulmonary embolism were compared with their ventilation-perfusion scans. The clinical presentation was consistent for the diagnosis of pulmonary emboli in all 55 patients. The scans were divided into the following four categories according to standard guidelines: normal, low, intermediate, and high probability. Three patients had normal scans confirmed by arteriography; 34 patients had high-probability scans, but only 22 (65 percent) were positive at arteriography. The remaining 18 patients had low or intermediate-probability scans (9 patients each), 5 of whom (28 percent) were positive at angiography (sensitivity 82 percent, specificity 57 percent). This study demonstrates the inaccuracy of ventilation-perfusion scanning for the evaluation of pulmonary embolus and may represent the variability of interpretation at individual institutions. Perhaps each institution may need to compare the results of ventilation-perfusion scanning and angiography to optimally select and treat patients.