A New Way to Dimer?

Written by Alex Chen, MD

Spoon-Feed
In this retrospective study, age-adjusted, clinical probability-adjusted, and standard D-dimer approaches had similar NPVs: 99.7%, 99.1%, 100%, respectively. Clinical probability-adjusted D-dimer has potential to exclude PE in more patients without imaging, but use caution before applying this in practice until prospectively validated.

Why does this matter?
The number of CT scans that are performed in the ED looking for PE continues to increase. This leads to increased cost and radiation exposure to patients. Age-adjusted D-dimer is a well validated, safe approach that allows more patients to be ruled out without imaging. What if we accepted a higher D-dimer cutoff for lower pretest probability patients?

More than one way to skin a cat… scan
This was a retrospective chart review of 1075 patients from 2 EDs in Canada. They were included if they had a Wells’ score of ≤6 and a D-dimer, CTPA, or VQ scan ordered to investigate PE. The primary outcome was exclusion of PE at initial presentation and 30-day follow-up with each D-dimer interpretation strategy. For the age-adjusted D-dimer cutoff they used age x 10 from 50-80 years. For the clinical probability-adjusted approach they used a cutoff of 1000 if the Wells’ score was ≤4 and 500 if the score was 4.5-6. The standard approach utilized a cutoff of 500 for all patients with a score ≤6. Here is how each cutoff performed.

 
 Adapted from cited article

Adapted from cited article

 

There were a number of weaknesses with this study. Besides the retrospective nature, 13/66 of the patients that were excluded (due to no D-dimer ordered) had a Wells’ score ≤6 and were found to have PE. Considering that only 38 patients (3.5%) had a PE, this could potentially throw the NPV out of the acceptable <2% zone. Also, it would have been very difficult to retrospectively determine from chart review the “Alternative diagnosis is less likely than PE” variable of the Wells’ score, which is based on subjective gestalt. Clinical-probability adjusted D-dimer appeared promising in this study, but caution is warranted before applying this in practice pending prospective validation.

Source
Comparison of the age-adjusted and clinical probability-adjusted D-dimer to exclude pulmonary embolism in the emergency department. Am J Emerg Med. 2018 Jul 30. pii: S0735-6757(18)30633-8. doi: 10.1016/j.ajem.2018.07.053. [Epub ahead of print]

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Reviewed by Clay Smith

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