D‐dimer Interval Likelihood Ratios for Pulmonary Embolism

Objective The objective was to estimate D‐dimer interval likelihood ratios ( iLR s) for diagnosing pulmonary embolism ( PE ). Methods The authors used pooled patient‐level data from five PE diagnostic management studies to estimate iLR s for the eight D‐dimer intervals with boundaries 250, 500, 750, 1,000, 1,500, 2,500, and 5,000 ng/ mL . Logistic regression was used to fit the data so that an interval increase corresponds to increasing the likelihood ratio by a constant factor. Results The iLR for the D‐dimer interval 1,000–1,499 ng/ mL was essentially 1.0 (0.98 with 95% confidence interval [ CI ] = 0.82–1.18). In the logistic regression model, the constant between‐interval factor was 2.0 (95% CI  = 1.9–2.1). Using these iLR estimates, if the pre–D‐dimer probability of PE is 15%, only a D‐dimer less than 500 ng/ mL will result in a posttest probability below 3%; if the pretest probability is 5%, the threshold for a “negative” D‐dimer is 1,000 ng/ mL . Conclusions A decision strategy based on these approximate iLR s agrees with several published strategies.