Clinical Validity of a Normal Perfusion Lung Scan in Patients: DISCUSSION part 2
The perfusion lung scan was of substantial clinical utility in our setting; 36 percent of patients referred with suspected pulmonary embolism had normal lung scans. These patients were spared further investigation and treatment for venous thromboembolism. The frequency of normal perfusion scans in patients with suspected pulmonary embolism ranges from 13 to 49 percent. The utility of perfusion scanning therefore depends on the clinical setting.
Many of the patients in the current study were outpatients at the time of presentation with suspected pulmonary embolism (361 of 515 patients [70 percent]). Since only one of the 361 outpatients (0.3 percent) with normal lung scans had symptomatic venous thromboembolism on follow-up, we conclude that it is safe to withhold anticoagulant therapy in outpatients with suspected pulmonary embolism who have a normal perfusion scan.
With knowledge of normal findings by perfusion lung scanning, an alternative diagnosis was pursued and established in 71 percent of our patients (367 of 515 patients). A wide variety of alternative diagnoses were found (Table 2). In a considerable proportion (29 percent) of patients, however, the cause of symptoms remained uncertain.
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In our study, the patients had a broad spectrum of clinical characteristics (Table 1) and a wide variety of comorbid conditions (Table 2). Patients were referred from both the community and hospital settings. For these reasons, our patients should be representative of those presenting to many other community- and hospital-based practices. Even in those hospital settings where the frequency of a normal perfusion scan is low (eg, 13 percent), it is still useful. The finding of a normal perfusion scan rules out clinically important pulmonary embolism and makes pulmonary angiography unnecessary. The perfusion scan remains the pivotal test in patients with suspected pulmonary embolism.