Clinical Validity of a Normal Perfusion Lung Scan in Patients
The perfusion lung scan is a pivotal test in the diagnosis of pulmonary embolism. In this test, which reflects the relative distribution of pulmonary blood flow, intravenously injected particles of radioactive macroaggregated albumin lodge in the pulmonary microcirculation. The distribution of these particles is measured by scanning the chest with a gamma camera in several projections. Studies in experimental animal models indicate that this technique detects 97 percent of occlusive emboli larger than 2 mm in diameter, but may fail to detect smaller pulmonary emboli or incompletely occlusive emboli.
It is generally accepted clinically that a normal perfusion scan excludes the presence of clinically important pulmonary embolism. This concept needs to be confirmed because the reported 95 percent confidence limits on the frequency of a false-negative perfusion scan result are broad.
The effectiveness of a diagnostic test can be readily assessed by long-term follow-up if (a) the disease studied is responsive to treatment, and (b) inadequate management produces clinical complications that can be measured objectively. These clinical complications can be used as endpoints to assess the validity of negative test findings. These requirements are met in a study of pulmonary embolism since (a) anticoagulant therapy is effective and (b) inadequate management results in recurrent venous thromboembolic events that can be readily measured. Viagra Professional
We have performed a prospective study of 515 consecutive patients with clinically suspected pulmonary embolism and normal perfusion lung scans. The objective was to test the safety of withholding anticoagulant treatment in such patients, regardless of the clinical manifestations. If the perfusion lung scan is rarely falsely normal, then venous thromboembolic events on follow-up should be very infrequent.