The indeterminate lung scan: its characteristics and its association with pulmonary embolism.

In an unselected series of patients whose only perfusion abnormalities on lung scan matched abnormalities on chest radiograph, the prevalence of pulmonary embolism ranged from 8 to 15%. These estimates depended upon whether only angiographic and autopsy-proven diagnoses were allowed or whether clinical diagnoses were also considered correct. Physical findings and laboratory data, in general, did not contribute to an elucidation of the cause of an indeterminate lung scan. Several radiographic findings with associated perfusion abnormalities (atelectasis alone, multiple infiltrates, bilateral effusions) were not seen in patients with pulmonary embolism. The probability of pulmonary embolism in patients with subsegmental defects alone and matching radiographic findings is about 9%; angiography in these patients would require that nearly two-thirds of the initial patient population undergo this procedure. Patients with larger defects (segmental or lobar) had a higher probability for pulmonary embolism (20--30%) and in these patients angiography is recommended.