Post-Test Probability Calculator

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The probability that the patient has the disease before the test is run. Often estimated from population prevalence, clinical risk scores, or physician judgment.
%
The probability that the test is positive given that the patient truly has the disease. Also called the true positive rate or recall.
%
The probability that the test is negative given that the patient truly does not have the disease. Also called the true negative rate.
%
A reference cohort size used to compute expected counts of true positives, false positives, true negatives, and false negatives. Does not affect probabilities.
people
Post-test probability (positive result)Moderate PPV
66.7%

Probability of disease given a positive test result (PPV)

Post-test probability (negative result)0%
Positive likelihood ratio (LR+)18
Negative likelihood ratio (LR-)0.105
Positive predictive value (PPV)0.7%
Negative predictive value (NPV)1%
Overall accuracy0.9%
True positives (TP)90
False positives (FP)45
True negatives (TN)855
False negatives (FN)10
66.7% PPV
Very low PPV<20%Low PPV20%-50%Moderate PPV50%-70%High PPV70%-90%Very high PPV90%+

PPV 66.7%, NPV 98.8% at 10% prevalence.

  • A positive result means there is a 66.7% chance the patient actually has the condition (PPV). Of every 100 people who test positive, about 67 truly have it.
  • A negative result means there is only a 1.2% chance the patient still has the condition. The NPV is 98.8%, so the test is highly reassuring when negative.
  • The positive likelihood ratio is 18.00, which is strong evidence for disease when the test is positive (LR+ above 10 is considered a large shift in probability).

Next stepUse post-test probability to guide the next diagnostic or treatment step. If the probability after a positive result is still below your action threshold, a second test or specialist referral may be warranted.

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