Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

Finding (Sign/Symptom/Lab/Study)Number Needed to Diagnose
(Positive Likelihood Ratio)
Capillary refill time6.9×
Sunken eyes3.4×
Speech not clear or expressive3.1×
Dry axilla2.8×
Upper or Lower extremity weakness2.3×
Dry tongue 2.1×
Dry mouth and nose m.m.2.0×
Tongue longitutinal furrows2.0×
Pulse change > 30bpm1.7×
Postural hypotension (SVP decr. > 20 mmHg)1.5×

Negative Findings (Patient Doesn't Have This)

Finding (Sign/Symptom/Lab/Study)Number Needed to Diagnose
(Negative Likelihood Ratio)
Dry mouth and nose m.m.0.3×
Tongue longitutinal furrows0.3×
Sunken eyes0.5×
Speech not clear or expressive0.5×
Dry tongue0.6×
Dry axilla0.6×
Capillary refill time0.7×
Upper or Lower extremity weakness0.7×
Pulse change > 30bpm0.8×
Postural hypotension (SVP decr. > 20 mmHg)0.9×



  • Determination of some of the physicial findings such as axillary moisture and capillary refill are very operator dependent and have a poor inter-rater reliability (in case of 'dry axilla" the kappa is 0.5)

  • Vital signs or postural changes have poor sensitivity in identifying blood loss or hypovolemia.


Rodrigo Kong, MD


March 15, 2012

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!)