Ventilation-perfusion (V/Q) lung scans are routinely used as the initial diagnostic test for ruling out a pulmonary embolism. Unfortunately, even under ideal circumstances, this test may not be diagnostic. A high-probability V/Q scan in the right clinical setting (high or moderate pretest probability) is considered diagnostic and usually will result in anticoagulation therapy. The next study of choice is a lower extremity venous ultrasound. If results of this test are negative, the patient typically has a pulmonary angiogram, which is considered diagnostic for pulmonary embolism. Meyerovitz and colleagues conducted a retrospective review of medical records to identify the prevalence of pulmonary embolism in patients with a low-probability V/Q scan, negative results on lower extremity venous ultrasonography and a high pretest suspicion for pulmonary embolism.

Medical records of patients who underwent pulmonary angiography to confirm a diagnosis of pulmonary embolism during a three-year period were reviewed. These records were further screened to select all patients whose V/Q scans were interpreted as low probability for pulmonary embolism but who underwent subsequent bilateral lower extremity venous ultrasonography. The latter study consisted of compression sonography of the popliteal, superficial femoral and common femoral veins, along with Doppler response to respiratory variation and calf compression.

A total of 365 patients underwent pulmonary angiography, including 62 with low-probability V/Q scans and negative lower extremity ultrasound studies. Of these patients, five (8 percent) had a pulmonary embolism, and 57 (92 percent) did not. The calculated 95 percent confidence intervals for this finding range from 2.7 to 18 percent. All five patients were ultimately treated with heparin and/or warfarin, and in two patients an inferior vena cava filter was inserted. There were no deaths related to pulmonary embolism in this subset of patients.

The authors conclude that patients with high pretest probability of pulmonary embolism but a low-probability lung scan and a negative lower extremity venous ultrasound examination need further diagnostic testing. Previous studies have found that only 50 percent of patients with a definite pulmonary embolism have positive results on lower extremity venous ultrasonography. In this study, positive results were found in only 20 percent of patients, leaving a large number of patients in this study without a definitive diagnosis. Pulmonary angiography should be used as the definitive diagnostic test.