Comprehensive Calculator Guide
This calculator turns a test’s 2×2 table (TP, FN, FP, TN) into sensitivity, specificity, PPV, NPV, accuracy and prevalence. With 90, 10, 45 and 855: sensitivity 90%, specificity 95%, but PPV only 66.7%.
Test vs patient
Sensitivity and specificity describe the test; PPV and NPV, which change with prevalence, tell a patient what a result means. Sensitive tests rule out; specific tests rule in.
How to Use
- Enter TP and FN.
- Enter FP and TN.
- Press “Calculate” to see all six measures.
Formula Used
Sens = TP/(TP + FN) | Spec = TN/(TN + FP) | PPV = TP/(TP + FP) | NPV = TN/(TN + FN)
Practical Examples
- Example 1
- 90, 10, 45, 855: 90%, 95%, PPV 66.67%.
- Example 2
- 70, 30, 9, 891: 70%, 99%, PPV 88.61%.
Important Tips
- Sensitive to rule out, specific to rule in.
- Don’t compare PPVs across prevalences.
- Use the post-test calculator for another prevalence.
Common Mistakes to Avoid
- Swapping FN and FP.
- Judging by accuracy alone.
- Treating PPV as fixed.
Frequently Asked Questions
- Sensitivity vs specificity?
- Catching the ill vs clearing the healthy.
- Why is PPV lower?
- False positives among the many healthy.
- What is accuracy?
- All correct results over the total.