D-Dimer, It’s Not Just a Cut-Off

It’s certainly simpler to have a world where everything is black or white, right or wrong, positive or negative. Once upon a time, positive cardiac biomarkers meant acute coronary syndrome – now we have more information and shades of grey in between. The D-dimer, bless its heart, is probably like that, too.

This is a simple study that pooled patients from five pulmonary embolism studies to evaluate the diagnostic performance characteristics of the D-dimer assay. Conventional usage is simply to deploy the test as a dichotomous rule-out – a value below our set sensitivity threshold obviates further testing, while above consigns us to the bitter radiologic conclusion. These authors, perhaps anticipating a more sophisticated diagnostic strategy, go about trying to calculate interval likelihood ratios for the test.

Using over 6,000 patients as their substrate for analysis, these authors determine the various likelihood ratios for D-dimer levels between 250 ng/mL and greater than 5,000 ng/mL, and identify intervals of gradually increasing width, starting at 250 and building up to 2,500. Based on logistic regression modeling, the fitted and approximate iLR range from 0.0625 for those with D-dimer less 250 ng/mL and increasing to 8 for levels greater than 5,000. Interestingly, a D-dimer between 1,000 and 1,499 had an iLR of roughly 1 – meaning those values basically have no effect on the post-test likelihood of PE.

The general implication of these data would be to inform more precise accounting of the risk for PE involving the decision to proceed to CTPA. That said, with our generally inexact tools for otherwise estimating pretest likelihood of disease (Wells, Geneva, gestalt), these data are probably not quite ready for clinical use. I expect further research to develop more sophisticated individual risk prediction models, for which these likelihood ratios may be of value.

“D-Dimer Interval Likelihood Ratios for Pulmonary Embolism”
https://www.ncbi.nlm.nih.gov/pubmed/28370759

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