Likelihood ratios

Likelihood Ratios and Bedside Estimates

Likelihood Ratio Approximate Change in Probability (%)

Values between 0 and 1 decrease the probability of disease

0.1 −45%

0.2 −30%

0.3 −25%

0.4 −20%

0.5 −15%

1 - 0%

Values greater than 1 increase the probability of disease

2 +15%

3 +20%

4 +25%

5 +30%

6 +35%

7 8 +40%

9 10 +45%

An easy way to recall at the bedside by simply remembering 3 specific LRs 2, 5, and 10—and the first 3 multiples of 15 (i.e., 15, 30, and 45). An LR of 2 increases probability 15%, one of 5 increases it 30%, and one of 10 increases it 45%. For those LRs between 0 and 1, the clinician simply inverts 2, 5, and 10 (i.e., 1/2 = 0.5, 1/5 = 0.2, 1/10 = 0.1). Just as the LR of 2.0 increases probability 15%, its inverse, 0.5, decreases probability 15%. Similarly, an LR of 0.2 (the inverse of 5) decreases probability 30%, and a LR of 0.1 (the inverse of 10) decreases it 45%. These benchmark LRs can be used to approximate the remainder of Table 1.

Although this method is inaccurate for pretest probabilities less than 10% or greater than 90%, this is not a disadvantage, because these polar extremes of probability indicate diagnostic certainty for most clinical problems, making it unnecessary to order further tests (and apply additional LRs).

J Gen Intern Med. 2002 August; 17(8): 647–650.