Comprehensive Calculator Guide
This calculator gives the chance of disease after a positive or negative test from the pre-test probability, sensitivity and specificity. With 1% prevalence and a 95% test, a positive result means only about 16%.
The false-positive paradox
Of 1,000 people, 9.5 true positives meet 49.5 false ones, so only 16.1% of positives are real. With a 40% pre-test probability the same test gives 92.7%. Likelihood ratios measure the test itself.
How to Use
- Enter the pre-test probability in %.
- Enter sensitivity and specificity in %.
- Press “Calculate” to see both post-test probabilities and the likelihood ratios.
Formula Used
PPV = sens × pre ÷ (sens × pre + (100 − spec)(100 − pre)) | LR+ = sens ÷ (100 − spec)
Practical Examples
- Example 1
- 1%, 95%, 95%: 16.1% after a positive.
- Example 2
- 40% pre-test: 92.7%.
Important Tips
- Ask for a confirmatory test.
- Use your doctor’s estimate if you have symptoms.
- LR+ above 10 is strong.
Common Mistakes to Avoid
- Reading sensitivity as PPV.
- Entering decimals instead of %.
- Ignoring the pre-test probability.
Frequently Asked Questions
- Sensitivity vs PPV?
- PPV depends on prevalence.
- Why repeat a positive?
- Many positives for rare diseases are false.
- Does it replace a doctor?
- No.