When Positive D-Dimers are Negative

This is the latest article from Jeff Kline, published in Thrombosis and Haemostasis (don’t you all subscribe to that, too?), concerning pulmonary embolism and d-Dimer.

Wouldn’t it be great if the d-Dimer wasn’t a dichotomous cut-off?  Where, if a patient were of sufficiently low pre-test probability, a d-Dimer value that was nearly negative still contributed adequately to a negative likelihood ratio to reduce the probability of a significant pulmonary embolism?  Well, that’s the theory behind this article – which looks at d-Dimer measurements combined with age, Wells’ score, and Revised Geneva scores.

There are a lot of complex tables in this article breaking down the various potential cut-off values for d-Dimer along with different pre-test probabilities, and the concept presented is that potentially higher cut-off values of d-Dimer can be used without missing PEs larger than sub-segmental.  This is presented in context that a higher cut-off might allow reductions in imaging, which seems fair.

However, the most interesting thing in this article to me is Figure 3 – which is d-Dimer concentration compared with fractional obstruction of pulmonary vascular tree.  It is, unfortunately, pretty clear there’s not a great linear relationship between dimer and pulmonary obstruction.  Most low d-Dimers had < 5% obstruction of the vascular tree, but at least one patient with a “negative” d-Dimer had 20% obstruction.  Beyond that, patients were just as likely to have 90% obstruction with modestly elevated d-Dimers than with massively elevated d-Dimers.

“D-dimer threshold increase with pretest probability unlikely for pulmonary embolism to decrease unnecessary computerized tomographic pulmonary angiography”
www.ncbi.nlm.nih.gov/pubmed/22284935