Evaluation of therapeutic decisions in patients with suspected pulmonary embolism and a low probability lung scan: application of Bayes' Theorem.

Three hundred and one consecutive lung scans were reviewed to determine the frequency of the different ventilation perfusion scan patterns and their use in the management of suspected pulmonary embolism. Based on the original report, an accurate diagnosis was possible in 99 patients (33%). Within this group pulmonary embolism was excluded by a normal lung scan in 35 (12%), and confirmed in 64 (21%) patients having a high probability pattern. There were 194 (64%) patients with a low probability scan, but in only 24 of these (12%) was venous thromboembolism objectively confirmed or excluded by additional investigations. Although the low probability scan is non-specific, the scan result was used as the sole objective test to determine therapy in 170 patients (56%). The clinical decision was to withhold anticoagulants in 143 patients and to treat 27 patients. Probability estimates using Bayes' Theorem have been applied to evaluate these management decisions.