If a diagnostic test has LR- < 1, what does it signify when the test is negative?

• A. It increases the odds of disease.
• B. It decreases the odds of disease.
• C. It has no effect on the odds.
• D. It makes the disease almost impossible.

Answer: (B)

Likelihood ratios show how a test result changes disease odds. If the negative result has LR- less than 1, it lowers the post-test odds of having the disease. In other words, a negative result reduces your suspicion after testing.

This update is calculated as post-test odds = pre-test odds × LR-. Since LR- is less than 1, multiplying by it shrinks the odds, meaning the disease becomes less likely after a negative test. For example, if your pre-test probability is 50% (odds 1:1) and LR- is 0.2, the post-test odds become 0.2 (probability about 16-17%), a meaningful reduction.

So the correct interpretation is that a negative test decreases the odds of disease.