Introduction Patients attending the ambulatory pulmonary embolism (PE) clinic at the Glenfield Hospital are risk stratified into low, intermediate and high risk based on the BTS scoring.¹ Those with a low or intermediate pre-test probability go on to have a microlatex D-dimer assay and if this is greater than 0.5 ug/mL, imaging in the form of CTPA or VQ scan is carried out.

It has been suggested that using a higher cut off value of D-dimer may improve specificity without affecting sensitivity for a PE.

Methods Data was collected for 2139 consecutive patients who presented to the ambulatory PE clinic between June 2010 and Dec 2014. For each of these patients, age, BTS clinical probability, D-dimer results and final diagnosis was recorded.

Receiver operating characteristics (ROC) curve analysis was performed separately for patients with low and intermediate probability, and the optimum cut-off value to exclude PE determined.

Results Of the 2139 patients, prevalence of PE was 3.2% (50/1535) in the low, 14.2% (63/443) in the intermediate and 26% (42/161) in the high probability group. No patients with a D-dimer of <0.5 ug/mL who were discharged without any radiological investigations have returned with a missed diagnosis of PE.

ROC curve analysis showed the optimum D-dimer cut off value in low risk patients was 0.52, and 0.57 in patients with an intermediate risk.

Conclusion The optimum D-dimer value must be chosen in the context of missed PEs versus scanning fewer people and thus avoiding unnecessary radiation and using resources more efficiently. A higher D-dimer of 1.0 ug/mL would have correctly avoided 161 scans and subsequently saved over £19,000.² This must be offset however against patients being incorrectly diagnosed and often ending up in hospital with complications. Using the same cut off of 1.0 ug/mL would have missed a total of 22 PE’s during the study period. Based on this the conventional D-dimer cut off value of 0.5 ug/mL is most appropriate for patients attending the ambulatory PE clinic.

References 1 Venous Thromboembolic diseases: the management of venous thromboembolic diseases and the role of thrombophilia testing. Costing report. NICE, June 2012

2 British Thoracic Society guidelines for the management of suspected acute pulmonary embolism. Thorax 2003;58:470-484