Positive Findings (Patient Has This)

Finding (Sign/Symptom/Lab/Study)Number Needed to Diagnose
(Positive Likelihood Ratio)
High Sensitivity D-Dimer with Low Pre-Test Probability2.4×
High Sensitivity D-Dimer with Mod Pre-Test Probability1.7×
High Sensitivity D-Dimer with High Pre-Test Probability1.5×

Negative Findings (Patient Does't Have This)

Finding (Sign/Symptom/Lab/Study)Number Needed to Diagnose
(Negative Likelihood Ratio)
High Sensitivity D-Dimer with Mod Pre-Test Probability0.05×
High Sensitivity D-Dimer with High Pre-Test Probability0.07×
High Sensitivity D-Dimer with Low Pre-Test Probability0.10×

Source: Wells PS, Owen C, Doucette S, Fergusson D, Tran H. Does this patient have deep
vein thrombosis? JAMA. 2006 Jan 11;295(2):199-207. Review. PubMed PMID: 16403932.

Narrative:

  • All LR's above are composite
  • Study Selection: Studies were selected for inclusion in the systematic review if they enrolled consecutive, unselected outpatients with suspected DVT and applied clinical prediction rules before D-dimer testing or diagnostic imaging.
  • Wells' criteria used as clinical prediction rule
  • Diagnostic accuracy for DVT improves when clinical probability is estimated before diagnostic tests.
  • When combined with a negative mod sensitivity D-dimer result, diagnostic imaging and anticoagulant therapy can be safely withheld for patients with a low clinical probability estimate since the negative LR (0.20; 95% CI, 0.12-0.31) is such that the probability after testing for DVT is less than 1%.
  • When combined with a negative high sensitivity D-dimer result, diagnostic imaging and anticoagulant therapy can be safely withheld in patients with a low (LR, 0.10; 95% CI, 0.03-0.37) or moderate clinical probability estimate (LR, 0.05; 95% CI, 0.01- 0.21) because they create a probability estimate after testing for DVT of less than 1%.

Caveats:

Author: Rodrigo Kong, MD

Published/Updated: March 15, 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!)