Friday, January 11, 2013

Who Are the PE Positive PERC Negatives?

This little letter, tucked away in the Correspondence section of Annals delves into the Pulmonary Embolism Rule-Out Criteria – a decision instrument of some controversy in Emergency Medicine.  Specifically, this letter addresses a case report from a previous issue of Annals of, essentially, a large pulmonary embolus diagnosed in a young patient who was otherwise PERC negative.

The authors from Carolinas Medical Center have a registry of 1,880 PE+ patients with which to evaluation, and they performed a retrospective application of the PERC rule.  Overall, 6% of this cohort was PE positive and PERC-negative.  When compared with the patients with PE who were PERC-positive, there are a few statistically significant differences – pleuritic chest pain was more common in PERC-negative patients with PE, along with pregnancy or post-partum status.  Unfortunately, these statistically significant relative differences reflect only small absolute differences of essentially clinically irrelevant magnitude.  The only mildly interesting tidbit from the letter is the statistic that none of PERC-negative PEs died within 30 days, compared with 5.7% of the PERC-positive cohort.

The authors suggest a couple weak clinical implications from the data, but these are limited by the retrospective nature of the analysis.  It is enough to remember that PERC-negative does not actually "rule-out" PE – it is simply a collection of negative likelihood ratios working against a pretest probability, resulting in clinical equipoise regarding the expect benefits vs. harms of CT pulmonary angiogram and the resultant harms of treatment in physiologically uninteresting PE.

"Clinical Features of Patients With Pulmonary Embolism and a Negative PERC Rule Result"
www.ncbi.nlm.nih.gov/pubmed/23260692

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