Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Visually "reduced" ejection fraction 4.1
≥3 B-lines in two bilateral lung zones 7.40
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)
Visually "reduced" ejection fraction 0.24
≥3 B-lines in two bilateral lung zones 0.16
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source: Martindale JL, Wakai A, Collins SP, et al. Diagnosing Acute Heart Failure in the Emergency Department: A Systematic Review and Meta-analysis. Acad Emerg Med. 2016;23(3):223-42.

Narrative: This was a large systematic review and meta-analysis (57 studies, n = 17,893) of the operating characteristics for diagnostic elements available to the emergency physician for diagnosing acute heart failure (AHF) including the history and physical, ECG, chest radiography, BNP/NT-proBNP (NPs), bedside echocardiography, lung ultrasound, and bioimpedance. They concluded that bedside lung US and echocardiography appear to the most useful tests for affirming the presence of AHF while NPs are valuable in excluding the diagnosis. Reduced ejection fraction was determined to have the highest +LR compared to other elements of the exam. However, the studied included in the final pooling appear to have only used "visual estimation" of reduced EF. With regards to lung US, a positive finding was defined in every study by the presence of at least three B lines in two bilateral lung zones.

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: Matthew Riscinti, 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!)