Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Presence of gallstone 7.48
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)
Presence of gallstone 0.16
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source: Ross M, Brown M, Mclaughlin K, et al. Emergency physician-performed ultrasound to diagnose cholelithiasis: a systematic review. Acad Emerg Med. 2011;18(3):227-35.

Narrative: This was a meta-analysis including 8 studies (n=710) evaluating the operating characteristics of emergency ultrasound (EUS) for identifying cholelithiasis in adult patients presenting to the emergency department with symptoms suggestive of biliary colic (RUQ pain, epigastric pain, or right flank pain). Reference standards included radiology performed ultrasound, CT, MRI, and/or surgical pathology. There was quite a variation in the technical ability between operators which may distort the pooled sensitivity and specificity.

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!)