Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
No lung sliding and lack of comet tails 50.5
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)
No lung sliding and lack of comet tails 0.09
Note: accuracy of ultrasound is operator-dependent. Reported LRs may not be reproducible by an inexperienced sonographer.

Source: Alrajhi K, Woo MY, Vaillancourt C. Test characteristics of ultrasonography for the detection of pneumothorax: a systematic review and meta-analysis. Chest. 2012;141(3):703-708.

Narrative: This was a systematic review of 8 prospective studies (n=1048) of adult patients. Included manuscripts evaluated for traumatic or iatrogenic pneumothorax. No studies that screened for spontaneous pneumothorax were included. Examiners were surgeons, radiologists, or emergency providers. Reference standard was pneumothorax found on CT or a rush of air upon tube thoracostomy. All studies but one used the ultrasonographic signs of lung sliding and comet tail to rule out pneumothorax. Although the exact technique used to perform the ultrasound examination is not reported with enough detail in some studies, most agree on requiring the examination of more than one intercostal space in both the midclavicular line and laterally and inferiorly at the anterior or midaxillary lines. Lastly, this data does not evaluate whether the pneumothoraces identified were clinically significant.

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: Kyle Kelson, 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!)