Evidence based practice
Diagnosis - calculating post-test probability for a specific patient
We often wants to know what the probability is that he or she has the disease, given a patient's test result. However, since baseline risk of the disease of our patient may be different from that in the study, the positive predictive value or negative predictive value quoted in the paper may not apply to our patients.

Fortunately, the values of sensitivity and specificity usually do not change with the baseline risks. So, we can usually apply the sensitivity and specificity values quoted in the paper to our patients. This means we need to calculate the probability that our patient have the disease ourselves.

To do this, we need to know the result of the test, the probability of our patient having the disease before the test, and the sensitivity and specificity taken from the paper.


Enter the sensitivity as a decimal (between 0 and 1):
Enter the specificity as a decimal (between 0 and 1):
Enter the pre-test probability as a decimal (between 0 and 1):
Post-test probability of patient with a positive test having disease as a decimal:
Post-test probability of patient with a negative test having disease as a decimal:


This page is created and maintained by Wai-Ching Leung
e-mail:- wp102@yahoo.com
last modified 17 September 2001