Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Dilated small bowel (diameter ≥25 mm) with peristaltic activity 27.18
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Negative Findings (Patient Doesn't Have This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
Dilated small bowel (diameter ≥25 mm) with peristaltic activity 0.08
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source: Gottlieb M, Peksa GD, Pandurangadu AV, Nakitende D, Takhar S, Seethala RR. Utilization of ultrasound for the evaluation of small bowel obstruction: A systematic review and meta-analysis. Am J Emerg Med. 2018;36(2):234-242.

Narrative: This was a meta-analysis including 11 studies (n=1178) to evaluate the test characteristics of US in diagnosis of small bowel obstruction (SBO). There was mild to moderate heterogeneity in diagnostic criteria, study location, sonographer experience, and reference standard. Specifically, most studies used 2.5 cm as the cutoff to diagnose SBO while one study used the cutoff of 3.0 cm, and several other studies only noted the presence of “dilated bowel loops” as a diagnostic criteria. Of the 11 studies included, only 3 were emergency department studies. Reference standards included surgery, clinical diagnosis, CT, or other advanced imaging. While there were multiple components to the index test and varied reference standard, this does appear to be the best and biggest review on this topic. These operating characteristics suggest ultrasound to be a valuable tool in the diagnosis of SBO, however further studies are needed specifically with regards to the emergency department setting. *Other diagnostic criteria included visualizing collapsed distal loops of bowel with decreased peristalsis

Caveats: Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Published in collaboration with The POCUS Atlas

Author: Roshanak Benabbas, MD

Published/Updated: September 13, 2018

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!)