Home
Back
Post Test Probability Formula:
| From: | To: |
Post Test Probability (PTP) is the updated probability of a condition after considering the results of a diagnostic test. It combines the pre-test probability with the test's likelihood ratio to provide a more accurate assessment of disease probability.
The calculator uses the PTP formula:
Where:
Explanation: The formula mathematically combines the pre-test probability with the test's diagnostic performance (LR) to calculate the updated probability after test results are known.
Details: Calculating post test probability is essential for clinical decision-making, helping clinicians determine whether to pursue further testing, initiate treatment, or rule out conditions based on test results.
Tips: Enter pre-test probability as a decimal between 0 and 1, and likelihood ratio as a positive number. Both values must be valid to calculate the post test probability.
Q1: What is pre-test probability?
A: Pre-test probability is the estimated probability of a condition before any diagnostic test is performed, based on prevalence, risk factors, and clinical presentation.
Q2: What is likelihood ratio?
A: Likelihood ratio quantifies how much a test result changes the probability of disease. LR > 1 increases probability, LR < 1 decreases probability.
Q3: How should I interpret the PTP result?
A: PTP values closer to 1 indicate high probability of disease, while values closer to 0 indicate low probability. Clinical decisions should consider the specific threshold probabilities for your context.
Q4: Can PTP be converted to percentage?
A: Yes, multiply the decimal result by 100 to get a percentage probability (e.g., 0.75 = 75% probability).
Q5: Are there limitations to this calculation?
A: The calculation assumes test characteristics are constant across populations and that pre-test probability is accurately estimated, which may not always be the case.