A new cancer screening test is evaluated in a population. The Likelihood Ratio Positive (LR+) is 15 and Likelihood Ratio Negative (LR−) is 0.04. Which interpretation is correct?

• A LR+ of 15 means the test barely changes the post-test probability
• B LR+ of 15 moderately increases post-test probability; LR− of 0.04 strongly rules out disease
• C A LR+ >10 provides a large, definitive change in probability and strongly rules in disease ✓
• D LR− of 0.04 should be interpreted as a 4% false-negative rate only

Explanation

Likelihood ratios summarise the discriminative ability of a test: LR+ = Sensitivity/(1−Specificity); LR− = (1−Sensitivity)/Specificity. An LR+ >10 is considered a strong result that substantially increases the probability of disease (rules in). An LR− <0.1 substantially decreases the probability (rules out). Here LR+ = 15 (strongly rules in) and LR− = 0.04 (strongly rules out). Both conditions being favourable indicates an excellent test. LR− ≠ false-negative rate; it is a ratio incorporating both sensitivity and specificity.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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