Now what would the situation be if Peter came from a low-risk population with a very low prevalence of HIV (for example, prevalence = 0.01 %)?

The contingency table would look like:

	Disease +	Disease -	Total
Test +	1	1	2
Test -	0	9998	9998
Total	1	9999	10000

 

 

 

 

 

 

Prevalence = 0.01%

Positive predictive value :
PVV = P (HIV + | Test +) = 1 / 2 = 50%

Conclusion - in case of low disease prevalence:

• there is a low probability of infection
• the test result has a 50% chance of being false positive
• patient may not be truly infected

Consequently, the confidence in the test result in addition to the test characteristics depends on the disease prevalence from which the tested individual was selected.