|Finding (Sign/Symptom/Lab/Study)||Number Needed to Diagnose(Positive Likelihood Ratio)|
|Beaded temporal artery||4.6×|
|Prominent or enlarged temporal artery||4.3×|
|Absent temporal artery pulse||2.7×|
|Tender temporal artery||2.6×|
|Any temporal artery abnormality||2.0×|
|ESR > 100 mm/h||1.9×|
|Optic atrophy or ischemic optic neuropathy||1.6×|
|ESR > 50 mm/h||1.2×|
|Any visual symptom||1.1×|
|Any funduscopic abnormality||1.1×|
|Unilateral visual loss||0.85×|
|Finding (Sign/Symptom/Lab/Study)||Number Needed to Diagnose(Negative Likelihood Ratio)|
|ESR > 50 mm/h||0.35×|
|Any temporal artery abnormality||0.53×|
|Prominent or enlarged temporal artery||0.67×|
|Absent temporal artery pulse||0.71×|
|Optic atrophy or ischemic optic neuropathy||0.8×|
|ESR > 100 mm/h||0.8×|
|Tender temporal artery||0.82×|
|Beaded temporal artery||0.93×|
|Any visual symptom||0.97×|
|Any funduscopic abnormality||1.0×|
|Unilateral visual loss||1.2×|
Caveats: It is unclear how these LR's are generalizable to the population at large.
Author: Rodrigo Kong, MD
Published/Updated: February 22, 2012
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!)