Positive Findings (Patient Has This)

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Negative Findings (Patient Doesn't Have This)

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Source: Barbic D, Chenkin J, Cho DD, Jelic T, Scheuermeyer FX. In patients presenting to the emergency department with skin and soft tissue infections what is the diagnostic accuracy of point-of-care ultrasonography for the diagnosis of abscess compared to the current standard of care? A systematic review and meta-analysis. BMJ Open. 2017 Jan 10;7:e013688.

Narrative: Over 3 million cases of skin and soft tissue infections (SSTI) including cellulitis and abscesses are managed in U.S. emergency departments (EDs) each year.1, 2, 3, 4 Overlap in presentations of cellulitis and abscess, which require different therapeutic approaches, has prompted increasing research into point-of-care ultrasound (POCUS) to help differentiate the two.1, 5, 6

The systematic review summarized here included prospective cohort studies evaluating POCUS for diagnosis of abscess in ED patients.7 The authors of the systematic review included patients with clinical evidence of SSTI. Reference standards varied, typically including draining purulent discharge, computed tomography scan, or clinical follow-up. There were no restrictions with regard to POCUS machine, transducer, protocol, or clinician background. The primary outcome was diagnostic accuracy for abscess in the ED.

The authors identified 8 relevant studies (n = 747 patients), with 3 conducted in adult ED and 5 in pediatric ED. Calculation of the point estimates for the diagnostic accuracy of POCUS found a sensitivity of 95.5% (95% confidence interval [CI] 88.9-98.3) and specificity of 80.3% (95% CI 56.4-92.7).8

Caveats: There are important limitations to the validity of these data. First, patients with cellulitis but initially negative POCUS for abscess may develop abscess later, confounding the reported results. Second, the included studies incorporated various gold standards for abscess diagnosis due to absence of a definitive criterion. Perhaps more importantly, this review included few studies, all with convenience samples, routine contamination between clinicians and sonographers (for both diagnosis and management decisions), and shifting reference standards.

These methodologic challenges tend to inflate sensitivity and specificity estimates, a concern highlighted by findings from both the largest study in the systematic review and a larger study published after the review.9, 10 The largest study included in the analysis, comprising 25% of the review’s sample size, found that POCUS did not add to the diagnostic post-test probability (and may have lowered both sensitivity and specificity) when clinicians felt confident of the diagnosis before ultrasound (i.e. when pre-test probability was high). However, when the pre-test probability was low or moderate, ultrasound was found to be helpful in increasing the post-test probability.9 Similarly, a large recent study reported that when clinicians felt certain (>90% of cases) of the diagnosis, ultrasound was unhelpful, while in most uncertain cases it improved accuracy.10

Based on this evidence, the accuracy numbers reported in the systematic review do not appear reliably valid for typical or common POCUS use in SSTI. We believe that the diagnostic accuracy of POCUS is dependent on the pre-test probability of abscess. POCUS does appear, however, to be potentially helpful in identifying abscess in ED patients in cases of diagnostic uncertainty. Therefore, we have assigned a color recommendation of Yellow (Unclear if benefits), though we recognize that POCUS is helpful in cases with clinical uncertainty after clinical examination.

The original manuscript was published in Academic Emergency Medicine as part of the partnership between TheNNT.com and AEM.

Author: Brit Long, MD; Alex Koyfman, MD; Michael Gottlieb, MD, RDMS
Supervising Editor: Shahriar Zehtabchi, MD

Published/Updated: November 20, 2019

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)