TATITIC CALCULATOR
Post Test Probability
A precise tool.
What is the Post Test Probability & How does it work?
The post‑test probability quantifies the chance that a condition is truly present after a diagnostic test result has been observed. It updates the pre‑test (or prior) belief using information about the test’s performance.
Bayes’ theorem provides the mathematical framework for this update. For a positive test result, the posterior probability is
\frac{Se \times P(D)}{Se \times P(D) + (1-Sp) \times (1-P(D))}
P(D|T+) = posterior probability of disease given a positive test
In practice, a high post‑test probability may prompt treatment, while a low value suggests further testing or observation. Understanding each component—prior probability, sensitivity (Se), and specificity (Sp)—is essential for accurate interpretation.
Parameters
Result
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Frequently Asked Questions
What is post-test probability?
Post-test probability is the updated chance that a condition is present after a diagnostic test result, incorporating both the test’s accuracy and prior belief.
How do I use this calculator?
Enter the pre-test probability, sensitivity, and specificity of the test to calculate the post-test probability for a positive or negative test result.
What does sensitivity mean in this context?
Sensitivity is the test’s ability to correctly identify those with the condition (true positive rate).
What does specificity mean?
Specificity is the test’s ability to correctly identify those without the condition (true negative rate).
Why is Bayes’ theorem used here?
Bayes’ theorem allows updating prior beliefs with new evidence, providing a more accurate assessment of disease probability after testing.
Can this calculator be used for any type of test?
Yes, it can be used for various diagnostic tests as long as you have the sensitivity, specificity, and pre-test probability.
What if I don’t know the pre-test probability?
Estimate it based on prevalence rates in similar populations or consult with a healthcare professional for guidance.
Results are for informational purposes only and do not constitute professional advice.