EBM notebook
Simplified calculations using likelihood ratios
Apneos Corporation
Belmont, California, USA
Key Words: evidence-based medicine
| The first 150 words of the full text of this article appear below. |
Likelihood ratios (LRs) have emerged as one of the most useful means to describe the implications of diagnostic test results. Unfortunately, many clinicians have difficulty relating likelihood ratios to the more familiar concepts of pretest and post-test probability, perhaps because the mathematics of LRs do not conveniently mesh with probabilities.1 Below I present a novel, integer-only algorithm useful in quickly appreciating the clinical implications of a likelihood ratio. The method is simple enough to be applied mentally, without recourse to calculators or other mechanical aids.
The algorithm computes the post-test probability (P) of disease as P = MI / (MI+NJ)—conveniently remembered as the "heart attack in New Jersey" equation. The table defines the terms of the equation. M and N derive from the pretest probability, while I and J derive from the likelihood ratio. "M" is conveniently remembered as the "Multiplier" that yields the pretest probability.
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