The Inappropriate Use of the SimpliRed D-Dimer Assay in the Diagnosis of Venous Thromboembolism,

Multiple diagnostic tests are available to evaluate patients with suspected venous thromboembolism (VTE). The diagnosis of VTE involves combining a clinical pre-test probability using validated scoring systems such as the Wells score (Wells, 2000) with laboratory testing and/or diagnostic imaging. The D-dimer assay is a highly sensitive screening test for VTE that has been shown to safely exclude VTE in patients with low pre-test probability (Kearon, 2006). This project was aimed at determining if the SimpliRed D-dimer assay is used appropriately at our institution. More specifically, we wanted to determine if it is being used in patients with low clinical probability of pulmonary embolism (PE) or deep vein thrombosis (DVT) based on the Wells score and in the absence of other variables which could falsely elevate the D-dimer as determined by expert opinion.

A retrospective review of 199 charts from St. Joseph's Hospital in Hamilton, Ontario from March 2007 to April 2007 was performed. Information was obtained on a standardized data collection form. Duplicate data extraction of the first 20 charts was undertaken to ensure concordance.

Of the 199 SimpliRed D-dimer assays ordered during this time period, 84 (42%) were inappropriate. Forty (48%) were considered inappropriate because they were ordered in patients with a moderate to high pre-test probability for VTE while 44 (52%) were ordered in patients with a co-existing medical condition that should have precluded the use of the assay including sepsis, pregnancy, active malignancy, and recent major surgery. The D-dimer assay was positive in 51 (61%) of the 84 inappropriate tests and 31 of these patients underwent further testing. Twelve patients were eventually diagnosed with VTE. The remaining 20 patients with a positive D-dimer ordered in an inappropriate context did not undergo further testing to investigate the positive result. Thirty-three (41%) patients with an inappropriately ordered D-dimer had a negative result and did not undergo further investigation. For patients with an appropriately ordered D-dimer assay (115 out of 199 or 58%), 96 had a negative D-dimer result. Of the 96, 28 patients went on to further testing including Doppler ultrasounds, ventilation/perfusion scans, or CT scans. One of the 28 patients had VTE.

This retrospective review revealed that 42% of the SimpliRed D-dimer assays ordered at our institution are inappropriate. Thirty three patients identified in this review who had a non-low pretest probability and negative D-dimer did not undergo further testing. A negative D-dimer result in these patients is not sufficient to rule out VTE – the lack of further appropriate testing could have led to missed VTE events. D-dimer assays, including the one used at our institution, have been shown to reliably rule out both DVT and PE in patients with a low pre-test probability (Kearon, 2006). At our institution 28 patients with an appropriately ordered, negative D-dimer underwent further testing that would not be recommended based on current diagnostic algorithms. These investigations are costly and expose the patient to possible unnecessary harm (such as radiation exposure from CT scans or use of anticoagulants until testing can be arranged). Of the patients with a negative D-dimer who went on to have further investigations, 1 was found to have VTE. This patient may have had a higher clinical pre-test probability than was assigned retrospectively from limited chart records. If the pre-test score assigned was accurate, this suggests that the failure rate of the D-dimer for ruling out VTE in a patient with a low pre-test probability is 3.6% (0.1% to 18.3%) in our series, higher than previously reported (Kearon, 2006).

Crowther:Pfizer: Consultancy, Research Funding; Leo Pharma: Consultancy, Research Funding; Bayer: Consultancy; CSL: Consultancy; BI: Consultancy, Research Funding; Behring: Consultancy; Octapharma: Consultancy; Artisan: Consultancy.