Risk of venous thromboembolism in patients with borderline quantitative D-dimer levels

Objective: The lower threshold for D-dimer in evaluating patients with low clinical risk of venous thromboembolism (VTE) ranges from 200 to 500 ng/mL. We compared the rates of VTE in patients based on D-dimer values. We hypothesized that the rate of VTE in low-risk patients with D-dimer levels less than 500 would be less than 1%. Methods: Study Design: This was a retrospective chart review: Setting: The study was performed in a academic, suburban emergency department (ED). Subjects: Emergency department patients with suspected VTE and D-dimer obtained were included in the study. D-dimer assay: The D-dimer assay is a quantitative instrumentation latex suspension of plasma specimens. Outcomes: Presence of VTE within 30 days of ED visit. Data Analysis: Assuming a 0% event rate in patients with D-dimer levels between 200 and 500 ng/mL, a sample of 450 patients would result in a 95% confidence interval upper limit of 0.6%. Results: There were 1270 ED patients with suspected VTE in which D-dimer levels were performed between October 2005 and October 2006. Patient mean age was 47.8 ± 19.3 years; 63.2% were female, 78.2% were white. Of all D-dimer levels, 497 (39.1%) were less than 200 ng/mL, 479 (37.7%) were between 200 and 500 μg/mL, and 294 (23.1%) were greater than 500 ng/mL. There were no VTE events diagnosed in any of the patients with D-dimer levels less than 200 ng/mL. Four patients with D-dimer levels between 200 and 500 μg/mL had a pulmonary embolism on computed tomography angiography. Of these 4 patients, 3 had moderate clinical risk based on Well's criteria and one had a false-positive computed tomography. There were no cases of VTE in the remaining 475 patients (0%; 95% confidence interval 0%-0.6%). Conclusion: The rate of confirmed VTE in low-risk patients with D-dimer levels between 200 and 500 ng/mL is very low. Low-risk patients with suspected VTE with D-dimer levels less than 500 ng/mL might not require additional testing.