The two key parameters in defining the utility of a screening test are its sensitivity and specificity. Sensitivity is the ability of the test to correctly identify individuals with the condition; specificity is the ability to correctly identify those without. There is usually a trade off between the two. For a screening test the priority is usually to identify all those with early or asymptomatic disease at the expense of including some false positives. This way those with a negative screening test can be told with some confidence that they are not affected, and those who screen positive can be further investigated through a diagnostic test with higher sensitivity to exclude the false positives. This traditional teaching relates to programmes to detect early stages of chronic disease such as cancers and cardiovascular disease.¹

In STIs, screening has an expanded role. The aim is not only to identify individuals with asymptomatic disease in …