What does LR+ quantify?

What does LR+ quantify?

• A. How much a positive result increases the probability of disease
• B. How much a negative result decreases the probability
• C. The overall prevalence
• D. The likelihood that the test is incorrect

Answer: (D)

LR+ measures how much a positive test result shifts the probability of having disease. It is the multiplier of the odds after a positive result, reflecting how much more likely you are to have the disease if the test is positive. In practice, you combine LR+ with the pre-test probability to obtain a post-test probability: convert the pre-test probability to odds, multiply by LR+, then convert back to a probability. A larger LR+ means a stronger ability to rule in disease with a positive result. For example, if LR+ is 4.5, a positive result makes the odds of disease 4.5 times higher than before. This concept is distinct from how much a negative result changes probability (that would be LR-), from disease prevalence, or from the overall accuracy or error rate of the test.

LR+ measures how much a positive test result shifts the probability of having disease. It is the multiplier of the odds after a positive result, reflecting how much more likely you are to have the disease if the test is positive.

In practice, you combine LR+ with the pre-test probability to obtain a post-test probability: convert the pre-test probability to odds, multiply by LR+, then convert back to a probability. A larger LR+ means a stronger ability to rule in disease with a positive result. For example, if LR+ is 4.5, a positive result makes the odds of disease 4.5 times higher than before.

This concept is distinct from how much a negative result changes probability (that would be LR-), from disease prevalence, or from the overall accuracy or error rate of the test.