1. What is Post-Test Probability?

Post-test probability is the updated probability of a condition after considering the results of a diagnostic test. It combines the pretest probability (initial likelihood before testing) with the test's likelihood ratio to provide a more accurate assessment.

2. How Does the Calculator Work?

The calculator uses the following formula:

Where:

• Pretest Odds — The initial odds of the condition before testing (odds = probability/(1-probability))
• LR — Likelihood Ratio (positive or negative depending on test result)

Explanation: The formula converts pretest odds to post-test odds using the likelihood ratio, then converts back to probability.

3. Importance of Post-Test Probability

Details: Understanding post-test probability helps clinicians interpret diagnostic test results in context, avoiding over- or under-estimation of disease probability based on test results alone.

4. Using the Calculator

Tips: Enter pretest odds (not probability) and likelihood ratio. Both values must be positive numbers. The result will be a probability between 0 and 1.

5. Frequently Asked Questions (FAQ)

Q1: How do I convert pretest probability to odds?
A: Odds = Probability/(1-Probability). For example, 20% probability = 0.2/(1-0.2) = 0.25 odds.

Q2: What's the difference between positive and negative LR?
A: Positive LR is used when test is positive, negative LR when test is negative. This calculator works with either.

Q3: What is a good likelihood ratio?
A: LR >10 or <0.1 generate large shifts in probability. LR 1 means no change in probability.

Q4: Can I use this for multiple tests?
A: Yes, by using the post-test probability from first test as pretest probability for next test.

Q5: How accurate is this calculation?
A: It's mathematically precise, but depends on accurate pretest probability and LR estimates.