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# Likelihood ratios

## 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

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