Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Cholelithiasis, wall thickening, pericholecystic fluid, sonographic Murphy's 15.6
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer. LR+ is a range but simplified here for the purposes of the interactive tool. See below for details.

Negative Findings (Patient Doesn't Have This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
Cholelithiasis, wall thickening, pericholecystic fluid, sonographic Murphy's 0.21
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer. LR- is a range but simplified here for the purposes of the interactive tool. See below for details.

Source: Jain A, Mehta N, Secko M, et al. History, Physical Examination, Laboratory Testing, and Emergency Department Ultrasonography for the Diagnosis of Acute Cholecystitis. Acad Emerg Med. 2017;24(3):281-297.

Narrative: This was a systematic review including 4 prospective studies evaluating the operating characteristics of bedside ultrasound for acute cholecystitis (AC) in adult patients seen in the emergency department with a clinical suspicion for AC or right upper quadrant pain. Sample size of the studies varied from 30 to 193 subjects. Reference standard was surgical pathology. The experience of the sonographers varied between the studies and in one study no documentation of sonographer experience was noted. There was significant heterogeneity across the included studies precluding the authors ability to pool the results hence a range is noted in the operating characteristics table.

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: Mike Macias, 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!)