Pulmonary embolism is a common medical condition that is difficult to diagnose. Many diagnostic tests are available to assist in establishing the diagnosis, but all of them have drawbacks, including poor sensitivity. The gold standard for diagnosis is angiography, but it is invasive and carries a 3 to 4 percent risk of cardiopulmonary complications. Helical computed tomographic (CT) scan has been used, but pulmonary embolism cannot be ruled out in patients with a negative scan. A newer test for pulmonary embolism, the assay for plasma D-dimer, is a promising exclusion test if the results are negative. The test originally was limited by intraobserver variation and the low sensitivity of the rapid latex tests. The newer tests for D-dimers are automated and more sensitive. Rathbun and colleagues evaluated the usefulness of an automated quantitative D-dimer test in the diagnosis of pulmonary embolism.
The trial was a prospective cohort study of consecutive patients admitted to two medical centers with suspected pulmonary embolism. Patients from the outpatient and inpatient areas who were sent for a ventilation-perfusion or helical CT scan were eligible for the study and were followed with an established protocol. Patients with a nondiagnostic lung scan or negative CT scan were enrolled in the study and had a D-dimer test and compression ultrasonography. Patients with a negative ultrasound test, a positive D-dimer test, and adequate cardiorespiratory reserve had repeat compression ultrasonography at five to seven days and again at 10 to 14 days. Anticoagulation therapy was withheld unless the ultrasound test was positive. Those with inadequate cardiorespiratory reserve were recommended for pulmonary angiography.
Of the 444 patients screened for the study, 125 met the inclusion criteria. The D-dimer test was negative in 14.4 percent of the 125 patients (11 of the 103 inpatients and seven of 22 outpatients). The D-dimer test was positive in all 11 patients who had a positive ultrasound test. Venous thromboembolism was confirmed in 14.6 percent of the cohort. Eleven percent of inpatients and 32 percent of outpatients had negative D-dimer results with a nondiagnostic lung scan or a negative helical CT scan.
The authors conclude that measurement of plasma D-dimer in inpatients who may have pulmonary embolism is of limited clinical value in patients who have a nondiagnostic lung scan or negative helical CT scan. They add that this may be because inpatients have a higher incidence of acute and chronic diseases that increase fibrin, which could alter the results of the D-dimer assay.