Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Free fluid in Morison's pouch 112
Free fluid in pelvis 9.5
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)
Free fluid in Morison's pouch 0.5
Free fluid in pelvis 0.47
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source: Moore C, Todd WM, O'brien E, Lin H. Free fluid in Morison's pouch on bedside ultrasound predicts need for operative intervention in suspected ectopic pregnancy. Acad Emerg Med. 2007;14(8):755-8.

Narrative: This was a prospective observational study (n=242) of patients presenting to the emergency department with suspected ectopic pregnancy, evaluating if free fluid in the peritoneal cavity identified by bedside ultrasound was predictive of the need for operative intervention. The included patients were suspected to have an ectopic pregnancy based on positive pregnancy test results, in their first trimester, with either abdominal pain or vaginal bleeding. All examinations were performed by emergency providers (EPs) using a transabdominal approach, to determine if free fluid was present in the hepatorenal space (Morison’s Pouch) and/or pelvis. Reference standard was chart review by one of four study investigators who were blinded to the EP performed US results. There was one patient who had free fluid in Morison’s Pouch but had a confirmed IUP (suspected to be a ruptured corpus luteum cyst) and no ruptured ectopic pregnancy. This study supports that free fluid present in Morison’s pouch in patients with suspected ectopic pregnancy strongly predicts the need for operative intervention.

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: Garrett Ghent, 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!)