1. What Are Likelihood Ratios?

Likelihood ratios (LRs) measure how much a diagnostic test result changes the probability of disease. LR+ indicates how much the probability increases with a positive test, while LR- shows how much it decreases with a negative test.

2. How Does the Calculator Work?

The calculator uses these formulas:

Where:

• \( Sensitivity \) — True positive rate (0-1)
• \( Specificity \) — True negative rate (0-1)

Interpretation:

• LR+ >10 or LR- <0.1 provide strong evidence
• LR+ 5-10 or LR- 0.1-0.2 provide moderate evidence
• LR+ 2-5 or LR- 0.2-0.5 provide weak evidence
• LR+ 1-2 or LR- 0.5-1 change probability little

3. Clinical Importance

Details: LRs are more useful than sensitivity/specificity alone because they can be directly applied to pre-test probabilities to calculate post-test probabilities.

4. Using the Calculator

Tips: Enter sensitivity and specificity as values between 0 and 1 (e.g., 0.95 for 95%). Both values must be valid (between 0-1, specificity not exactly 1).

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between LR+ and LR-?
A: LR+ applies when a test is positive, LR- when a test is negative. They answer different clinical questions.

Q2: How do I convert LRs to probabilities?
A: Use Fagan's nomogram or convert pre-test probability to odds, multiply by LR, then convert back to probability.

Q3: What are good LR values?
A: Higher LR+ and lower LR- are better. Ideal tests have LR+ >10 and LR- <0.1.

Q4: Can LRs be used with multiple tests?
A: Yes, multiply successive LRs assuming tests are conditionally independent.

Q5: What if specificity is 1?
A: LR+ becomes undefined (division by zero). Perfect specificity (100%) is rare in practice.