Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Saline bubbles in the right heart 100000
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer. LR+ is infinity; set as 100,000 for the purposes of this interactive tool.

Negative Findings (Patient Doesn't Have This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
Saline bubbles in the right heart 0.13
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source: Wilson SP, Assaf S, Lahham S, et al. Simplified point-of-care ultrasound protocol to confirm central venous catheter placement: A prospective study. World J Emerg Med. 2017;8(1):25-28.

Narrative: This was a prospective study (n=78) in critically ill patients in the emergency department (ED) and intensive care unit (ICU) at a single academic center, evaluating the use of POCUS to identify correct placement of supra-diaphragmatic central venous catheter (CVC) placement. All CVC placements and POCUS exams were performed by resident trainees. Correct positioning of the CVC was considered if turbulent flow was visualized in the right atrium on sub-xiphoid, parasternal, or apical cardiac ultrasound after injecting 5 cc of sterile, non-agitated, normal saline through the CVC. Reference standard was confirmatory chest radiography.

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; John F Kilpatrick, 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!)