Table 1 Illustration of predictive value of a positive test (PVP) in high and low prevalence populations.

Prevalence of chlamydia in population		10% (high)	2% (low)
True positives among 1000 women		100	20
True positives detected by the test (sensitivity  = 92.6%)		92	18
False positives given by the test (specificity  = 99.4 (0.6 false positives per 100)		6	6
Predictive value positive (PVP)  =  true positives/total positive results		92/98 (94%)	18/24 (75%)