What does a high LR+ indicate about diagnostic testing?

What does a high LR+ indicate about diagnostic testing?

• A. LR+ indicates that a negative result reduces the odds of disease.
• B. LR+ is calculated as Sp / (1 - Sn).
• C. A high LR+ meaningfully increases post-test odds after a positive result.
• D. LR+ is a measure of disease prevalence.

Answer: (C)

Think of diagnostic evidence as shifting pre-test odds into post-test odds. A positive test result multiplies those odds by the positive likelihood ratio (LR+). When LR+ is high, that multiplication is large, so the post-test odds become much higher, meaning the test provides strong evidence to rule in disease after a positive result. The correct statement captures this idea: a high LR+ meaningfully increases post-test odds after a positive result. Note why the others don’t fit: a negative result affects post-test odds via LR-, not LR+; the formula for LR+ is sensitivity divided by (1 − specificity), not specificity divided by (1 − sensitivity); and LR+ describes how a positive test changes probability, not disease prevalence.

Think of diagnostic evidence as shifting pre-test odds into post-test odds. A positive test result multiplies those odds by the positive likelihood ratio (LR+). When LR+ is high, that multiplication is large, so the post-test odds become much higher, meaning the test provides strong evidence to rule in disease after a positive result.

The correct statement captures this idea: a high LR+ meaningfully increases post-test odds after a positive result.

Note why the others don’t fit: a negative result affects post-test odds via LR-, not LR+; the formula for LR+ is sensitivity divided by (1 − specificity), not specificity divided by (1 − sensitivity); and LR+ describes how a positive test changes probability, not disease prevalence.