The was a systematic review including 20 prospective adult and pediatric studies (n=2513) with varied settings including the emergency department, inpatient wards, or ICU. A positive finding on ultrasound was identified as an alveolar and interstitial pattern or consolidation, although this is not further expanded upon. Gold standard was either CT, chest radiography, or “clinical diagnosis” depending on the study. One large caveat of this study is that it has a very large degree of heterogeneity, with ultrasound examinations performed by emergency physicians, intensivists, and radiologists of varying levels of expertise, on patients ranging from ambulatory to critically ill.
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 Kyle Kelson, MD
Published/Updated September 13, 2018
What are Likelihood Ratios?
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!)