P146 Can Raising the D-Dimer Threshold Safely Reduce the Number of CT Pulmonary Angiograms Performed in Suspected Pulmonary Embolism?
Introduction CT Pulmonary Angiography (CTPA) is the gold standard investigation for suspected pulmonary embolism (PE). Low or intermediate probability clinical prediction (e.g. Wells score) combined with a negative D-dimer effectively rules out PE in over 97% of cases, avoiding the need for CTPA and its inherent risks (radiation exposure and contrast induced nephropathy). This is the recommendation of the BTS guidelines. We undertook a study to examine if the BTS guidelines were being adhered to in our Trust, and whether increasing the D-dimer threshold may safely reduce the need for CTPA.
Methods We obtained a list of CTPAs performed within the Trust between September 2009 and September 2011 and searched our pathology system for a contemporary D-dimer result (HemosIL latex immuno-assay). For all patients with a negative D-dimer (≤230ng/mL), we looked for a documented pre-test probability score in the clinical notes or calculated a Wells score if not documented. We then analysed CTPA results with D-dimer between 230–500ng/mL.
Results There were 1645 CTPAs performed during the study period, of which 15% had confirmed PE 903(54.9%) had a contemporary D-dimer result, and of these 57(6.3%) had a negative D-dimer, and 193(21.4%) were between 230–500ng/mL. In the negative D-dimer group, 3 (5.3%) had confirmed PE’s on CTPA. One was on tranexamic acid, which can falsely lower D-dimer, and 2 had prolonged admissions in whom D-dimer testing was not appropriate. We were able to examine the notes of 39/57 cases and only 3 (8%) had a pre-test probability documented. On review 31/39 (79.5%) had a low to intermediate Wells score and should not have had a CTPA.
In the 230–500ng/mL D-dimer group, there were only 4/193 (2.1%) positive CTPAs with a negative predictive value 98%.
Conclusions In our Trust, the lack of pre-test probability scoring combined with D-dimer is leading to inappropriate CTPAs. The rate of PE in the patients with a D-dimer between 230–500ng/mL is also very low. A protocol recommending initial treatment pending an urgent respiratory team review prior to CTPA, could safely reduce the number performed in this group, if combined with a low to intermediate pre-test probability score.