The Sensitivity and Specificity of a Red Blood Cell Agglutination D-Dimer Assay for Venous Thromboembolism When Performed on Venous Blood
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BACKGROUND: Studies evaluating the accuracy of the SimpliRED D-dimer assay for venous thromboembolism (VTE) have used a capillary fingerstick blood sample, which requires the test to be performed immediately at the bedside. Initial studies showed a sensitivity for VTE of 90% to 95% when the assay was performed by a finite number of experienced health care workers. However, because of the test's subjectivity, misinterpretation of the result is possible when performed by inexperienced staff. Recent reports by other investigators indicated a low sensitivity of this assay for VTE and noted a reduction in sensitivity (84%) for pulmonary embolism. OBJECTIVE: To determine the sensitivity and specificity of the D-dimer test performed in the laboratory by experienced technologists on venous whole-blood samples in routine collection tubes. If D-dimer testing results accurately detect VTE when performed in this manner, concerns about the sensitivity of this assay would be solved. METHODS: One hundred forty-eight consecutive patients with suspected VTE underwent D-dimer testing at the bedside using a fingerstick sample and venous blood collected into a plain tube. Venous blood was also collected into tubes containing tri-potassium EDTA, sodium citrate, or a combination of lithium and heparin for D-dimer testing in the laboratory. In addition, the EDTA tube was refrigerated overnight at 4 degrees C for retesting at approximately 24 hours. The presence or absence of VTE was determined by means of objective results of testing and a 3-month follow-up. RESULTS: Thirty-four subjects (23%) had confirmed VTE (25 with deep vein thrombosis; 9 with pulmonary embolism). All laboratory venous blood D-dimer results showed sensitivities of 97%, specificities of 61% to 64%, and negative predictive values of 99%, compared with 88%, 71%, and 95%, respectively, when the results were obtained by means of fingerstick at the bedside. CONCLUSIONS: The SimpliRED D-dimer assay performed in the laboratory on venous blood, collected into any of 3 routine laboratory tubes, is sensitive and moderately specific for VTE. Based on this study, immediate bedside testing (particularly by inexperienced personnel) under suboptimal conditions is unnecessary. Furthermore, the high sensitivity of refrigerated EDTA samples allows specimens to be stored or transported (on ice at 4 degrees C) for testing for 24 hours after collection.
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