Likelihood Ratio
The Likelihood Ratio (LR) is the likelihood that a given test result would be expected in a patient with the target disorder compared to the likelihood that that same result would be expected in a patient without the target disorder. In other words, the likelihood ratio  (LR)  is the ratio of the probability of a specific test result in people who have a particular disease to the probability in people who do not.

The LR indicates how well the test rules in or rules out a given disease. A test with an LR of 1.0 indicates that it does not change the probability of disease. The higher above 1 the LR is, the more likely it is that the disease is present. An LR >10 is considered good.
Conversely,  the lower the LR is below 1, the more likely it is that the disease is not present. An LR <0.1  is considered good.

Likelihood ratios are an alternative way to summarise diagnostic accuracy.

Sample Question from ABFM: A new PE test has a likelihood ratio of 1. What conclusion can be made from this information? The test can neither confirm nor rule out pulmonary embolism.

Resource:
http://www.cebm.net/likelihood-ratios/ , Last Accessed 01/02/2016
The ABFM 2014 Intraining.

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