Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

CriteriaProbability of Disease (Positive Likelihood Ratio)
≥ 4 POUNDing Criteria5.8×

Negative Findings (Patient Doesn't Have This)

None identified.


  • Outcome: Migraine, defined by International Headache Society criteria (applied by a neurologist)

  • Level I Evidence Study: Independent, blinded comparisons of components of the clinical examination with a gold standard among 100 or more consecutive patients with headache.


Criteria developed based on a single study with Level I Evidence


Rodrigo Kong, MD and Shahriar Zehtabchi, MD


February 15, 2013

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!)