Introduction Pulmonary embolism (PE) is a common presentation in the emergency department and in-patient setting. Measurement of D-dimer in conjunction with clinical risk assessment is used to exclude patients at low risk of PE. Some of the conditions that mimic PE, including infection and inflammation, are also associated with elevated D-dimer concentrations such that the test lacks specificity. Most infectious and inflammatory conditions result in an elevated acute-phase serum response which can be quantified using C-Reactive Protein (CRP) assay. We hypothesised, therefore, that patients with isolated PE would have a higher D-dimer: CRP ratio than patients with infectious or inflammatory mimics of PE and therefore that this ratio would be more discriminatory.
Methods We analysed data from all patients who underwent V/Q scanning to confirm or exclude PE at Royal Free Hampstead NHS Trust, London, UK, during 2010. The CRP and D-dimer results closest, but preceding the V/Q scan were analysed using receiver operator characteristic (ROC) curves to test the hypothesis that the D-dimer: CRP ratio (expressed as ng/ml:mg/l) was a better predictor or PE than D-dimer alone.
Results 179 patients (mean (SD) age 52.8 (19.7) years) had a V/Q scan for suspected PE during the study period. Of these, 85 had a D-dimer assay, a median (IQR) of 1 (0–1) days prior to the imaging. The median D-dimer concentration was 272 (178–675) ng/ml. 137 patients had CRP assay (12 (3–56) mg/l), measured 1 (0–1) days prior to imaging. It was possible to calculate a D-dimer: CRP ratio in 78 patients (44% of the total), of whom 19 (24%) had a V/Q scan reported as high risk for PE. D-dimer, and the D-dimer: CRP ratio, but not CRP were significantly higher between patients who did and did not have high-risk V/Q scans (Mann–Whitney U test analyses: 764 vs 245 ng/ml, p=0.001; 107 vs 31 units, p=0.020 and 20 vs 10 mg/l, p=0.134 respectively). Biomarker data were log10 transformed to permit ROC analysis. Area-under-curve (AUC) values using ROC for D-dimer alone, and D-dimer: CRP ratio were 0.74 and 0.68 respectively, both less than the standard criteria for utility of 0.8.
Conclusions D-dimer: CRP ratio is not superior to D-dimer alone in predicting PE in patients with a clinical suspicion of this diagnosis sufficient to require V/Q scanning.