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What do these mean?
Positive and Negative Likelihood Ratios
Positive Likelihood Ratio (LR+)
Definition: The positive likelihood ratio is the ratio of the probability of a positive test result in patients with the disease to the probability of a positive test result in patients without the disease.
Formula: LR+ = Sensitivity / (1 − Specificity)
Interpretation: A higher LR+ indicates that a positive test result is much more likely in patients with the disease compared to those without it. For instance, an LR+ of 10 means that a positive test result is 10 times more likely in a patient with the disease than in one without it.
Negative Likelihood Ratio (LR−)
Definition: The negative likelihood ratio is the ratio of the probability of a negative test result in patients with the disease to the probability of a negative test result in patients without the disease.
Formula: LR− = (1 − Sensitivity) / Specificity
Interpretation: A lower LR− indicates that a negative test result is much more likely in patients without the disease compared to those with it. For instance, an LR− of 0.1 means that a negative test result is one-tenth as likely in a patient with the disease as in one without it.
From likelihood ratio to post-test probability
Likelihood ratios update a pretest probability: post-test odds = pretest odds × LR. Try it:
Example: ST depression (LR+ 5.3) with a 13% pretest probability gives about 44%, the value the source review reports.
In the Calculate tab, findings with an LR above 1 are listed as pointing towards ACS and findings with an LR below 1 as pointing away from ACS, strongest first. Applying several LRs one after another assumes the findings are independent, which is rarely true at the bedside, so the tool reports each finding separately.