Likelihood ratios

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Likelihood Ratio Approximate Change in Probability (%)

LR Post-Test Probablility Change
9-10 45%
7-8 40%
6 35%
5 30%
4 25%
3 20%
2 15%
1 0%
0.5 -15%
0.4 -20%
0.3 -25%
0.2 -30%
0.1 -45%
  • Values <1 decrease the probability of disease
  • Values >1 increase the probability of disease

Comments[1]

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).

References

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