POCUS Atlas: Pediatric Appendicitis

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
Non-compressible appendix >6 mm 9.24

Negative Findings (Patient Doesn't Have This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
Non-compressible appendix >6 mm 0.17
Note: while the authors did find a negative LR of 0.17, they concluded that a positive ED‐POCUS is diagnostic but a negative ED‐POCUS is not enough to rule out AA. As always, use clinical judgment in interpreting these results in patients.


This systematic review and meta-analysis of 4 studies (n=461) evaluated the accuracy of emergency department POCUS performed by EM or PEM physicians for diagnosis of acute appendicitis in children. The main limitation of the study was the high prevalence of equivocal results. However, authors did a sensitivity analysis with and without equivocal cases and results were similar. A mathematical model was used to compare POCUS to CT scan and MRI. The authors concluded that a positive POCUS exam is diagnostic, obviating the need for CT, however if POCUS is equivocal or negative, further imaging with CT or MRI is necessary.


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

Published in collaboration with The POCUS Atlas


Roshanak Benabbas, 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!)