# 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

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).