Carpal Tunnel Syndrome

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

Finding (Sign/Symptom/Lab/Study)Number Needed to Diagnose
(Positive Likelihood Ratio)
Flick Sign  21.4×
Closed Fist Sign  7.3×
Hypalgesia 3.1×
Square Hand Sign  2.7×
Classical or Probable (Hand diagram, JAMA 2000)2.4×
Weak Thumb Abduction 1.8×
Thenar Atrophy 1.6×
Abnormal Vibration 1.6×
Abnormal Monofilament 1.5×
Bilateral Symptoms 1.4×
Tinnel Sign 1.4×
Phalen Sign 1.3×
Age > 40 years 1.3×
2 point discrimination 1.3×
Nocturnal Paresthesia 1.2×
Pressure Provocation Test 1.0×
Tourniquet Test 1.0×

Negative Findings (Patient Doesn't Have This)

Finding (Sign/Symptom/Lab/Study)Number Needed to Diagnose
(Negative Likelihood Ratio)
Flick Sign  0.1×
Closed Fist Sign  0.4×
Classical or Probable (Hand diagram, JAMA 2000) 0.5×
Age > 40 years 0.5×
Weak Thumb Abduction 0.5×
Square Hand Sign  0.5×
Bilateral Symptoms 0.7×
Nocturnal Paresthesia 0.7×
Abnormal Monofilament 0.7×
Hypalgesia 0.7×
Phalen Sign 0.7×
Abnormal Vibration 0.8×
Tinnel Sign 0.8×
Tourniquet Test 1.0×
Thenar Atrophy 1.0×
2 point discrimination 1.0×
Pressure Provocation Test 1.0×


This review addresses the accuracy of the history and physical examination in diagnosing CTS, as confirmed by electrodiagnostic studies.
The data are derived from symptomatic patients presenting to an orthopedic surgeon, physical therapist, or an electrodiagnostic laboratory.
There are no data addressing the value of physical diagnosis in patients presenting to a primary care physician/ ED with symptoms suggestive of CTS.



Rodrigo Kong, MD and Shahriar Zehtabchi, MD


January 9, 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!)