Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
≥3 B-lines in two bilateral lung zones 12.38
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)
≥3 B-lines in two bilateral lung zones 0.06
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source: Al deeb M, Barbic S, Featherstone R, Dankoff J, Barbic D. Point-of-care ultrasonography for the diagnosis of acute cardiogenic pulmonary edema in patients presenting with acute dyspnea: a systematic review and meta-analysis. Acad Emerg Med. 2014;21(8):843-52.

Narrative: This was a systematic review including 7 prospective case control or cohort studies (n=1075) evaluating the sensitivity and specificity of B-lines in diagnosing acute cardiogenic pulmonary edema (ACPE). The included studies recruited patients presenting to the hospital with acute dyspnea, or where there was a clinical suspicion of congestive heart failure. The setting was either the emergency department (ED) , ICU, or inpatient wards. Ultrasound examinations were performed by any non-radiologist physician. *Various lung ultrasound protocols were used, including the Volpicelli method, the Lichtenstein protocol, and the Comet Score. All involved using B-lines to make the diagnosis of ACPE. The varied protocols used for diagnosis may explain the increased sensitivity noted in this study compared to other meta-analysis. Gold standard was heterogeneous amongst studies with 'final diagnosis from clinical follow-up' being an acceptable reference standard.

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: Kyle Kelson, 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!)