Patients undergoing evaluation for possible pulmonary embolism (PE) often have a non-diagnostic ventilation-perfusion (V/Q) scan. Nondiagnostic scans are interpreted as low, moderate or indeterminate probability. These patients, therefore, commonly undergo a lower extremity venous ultrasound examination, with normal results indicating a decreased probability of PE. Daniel and associates evaluated the diagnostic utility of the lower extremity Doppler ultrasound scan in outpatients with suspected PE.

Patients with suspected PE but nondiagnostic V/Q scans followed by venous compression ultrasound examination of the lower extremities were eligible for the prospective study. Results were classified to identify the best possible sensitivity and specificity of the ultrasound examination in patients not undergoing pulmonary angiography. The study population was classified as true-positive or true-negative for PE based on the results of the ultrasound examination.

Of the 570 patients who underwent a V/Q scan in the emergency department, 156 met the study criteria. Twenty-one patients had positive findings on lower extremity ultrasound examination, while 135 had negative findings. Assuming all patients with positive findings had PE and those with negative findings did not, the best-case sensitivity of ultrasound examination was 54 percent, and the best-case specificity was 98 percent.

The authors conclude that negative findings on a single lower extremity venous ultrasound examination in ambulatory patients with a nondiagnostic V/Q scan do not reduce the probability of PE to an acceptably low level. Rather, these results may suggest that PE in ambulatory patients originates less frequently in the proximal leg veins than was previously thought. It may also suggest that results of an ultrasound examination cannot accurately detect proximal thrombosis in patients who are asymptomatic. Further research is needed to better predict the presence of PE in patients with nondiagnostic V/Q scans and no evidence of deep venous thrombosis.