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Validity and utility of screening tests for STIs
  1. H Ward1,
  2. J Weber2
  1. 1Department of Epidemiology and Public Health, Imperial College London, UK
  2. 2Department Genitourinary Medicine and Communicable Diseases, Imperial College London, UK
  1. Correspondence to:
 Helen Ward, Department of Epidemiology and Public Health, Imperial College London, St Mary’s Campus, Norfolk Place, London W2 1PG, UK; 

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Implications for STI control

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 …

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