Study objective: To determine whether measurement of D-dimer, using an enzyme-linked immunosorbent assay (ELISA) with a cutoff of 300 ng/ml, and a latex agglutination assay with a cutoff of 500 ng/ml, is clinically useful in patients with suspected pulmonary embolism (PE).
Design: Prospective cohort.
Setting: Tertiary care referral center, university-affiliated hospital.
Patients: Two hundred twenty-one consecutive patients with clinically suspected PE.
Intervention: All patients had blood drawn to measure levels of D-dimer and underwent ventilation/perfusion (V/Q) lung scanning and bilateral impedance plethysmography (IPG); pulmonary angiography was performed in nine patients. Patients were classified as follows: (1) PE-positive; positive pulmonary angiography or high probability V/Q scan or non-high-probability V/Q scan and either abnormal IPG (either at presentation or on serial testing and confirmed by contrast venography) or symptomatic thromboembolic event within 3 months of presentation; or (2) PE-negative; normal V/Q scan or normal pulmonary angiography or non-high-probability V/Q scan and either normal serial IPG or abnormal IPG with normal venography and absence of symptomatic venous thromboembolism within 3 months of followup. Forty-three patients were classified as PE positive and 178 patients were classified as PE negative.
Measurements and results: The sensitivities, specificities, positive predictive values, and negative predictive values of the ELISA and latex agglutination assay were calculated for all patients and for the subgroup of patients with non-high-probability V/Q scans. The ELISA D-dimer, using a cutoff of 300 ng/ml, showed a sensitivity and negative predictive value of 100 percent in all patients and patients with non-high-probability V/Q scans, but the corresponding specificities were only 26 percent and 13 percent, respectively. The latex agglutination assay for D-dimer using a cutoff of 500 ng/ml showed the following: sensitivities of 84 percent in all patients and 90 percent in patients with non-high-probability scans, negative predictive values of 93 percent in all patients, and 98 percent in patients with non-high-probability scans and specificities of 56 percent in all patients and 55 percent in patients with non-high-probability scans.
Conclusions: This study demonstrates that an ELISA D-dimer result of less than 300 ng/ml excludes PE but occurs in a small proportion of patients with clinically suspected PE. The latex agglutination assay, using a cutoff of 500 ng/ml, has potential clinical utility in excluding PE in the subgroup of patients with clinically suspected PE and non-high-probability V/Q scans. However, the 95 percent confidence interval on the observed sensitivity of the latex agglutination assay in patients with non-high-probability V/Q scans is wide. Therefore, these promising results should be confirmed in a large clinical trial before the latex agglutination assay is used to make management decisions.