Rule Out Ectopic Pregnancy

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
Visualized IUP 0.08
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Negative Findings (Patient Doesn't Have This)



This was a meta-analysis including 10 studies (n=2057) examining the operating characteristics of bedside emergency provider (EP) performed ultrasound to rule out ectopic pregnancy. All studies used the visualization of an IUP (intrauterine pregnancy) on ultrasound as the rule out criteria. The ultrasound examinations performed by EPs included transabdominal, transvaginal or both. Reference standards included formal radiology US, gynecology US, over-read of EP performed US by radiology, or clinical record review. They did not report specificity and positive LR's because of significant heterogeneity in these results when data was pooled. This study supports the utility of EP performed ultrasound as a strong rule out test for ectopic pregnancy.


Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Published in collaboration with The POCUS Atlas


Garrett Ghent, MD


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!)