Numbers needed to test to find one infection

 Sexually Transmitted Infections (STIs) are transmitted across sexual networks through partner change and the majority of STIs are silent and do not produce signs or symptoms.  In many cases the person infected and possibly spreading infections is unaware they are an STI transmitter.  Testing for STIs is recommended when individuals have multiple partners or at a time of partner change and the tests are simple for individuals to do either via home testing, visiting a clinic or their family doctor.  A local testing service can be found using SXT (www.sxt.org.uk). 

The tests to do are shown in the table below:

Gender / Sexuality	Blood test (HIV & Syphilis)	Urine test (Chlamydia & Gonorrhoea)	Throat swab (Chlamydia & Gonorrhoea)	Anal swab (Chlamydia & Gonorrhoea)	Vaginal swab (Chlamydia & Gonorrhoea)
Male heterosexual	Yes	Yes
Woman	Yes		?	?	Yes
Man who has sex with men	Yes	Yes	Yes	Yes

? Testing the throat & rectum in women depends on the sexual history

These sensitive tests can accurately rule out an infection if they are done after what is known as the 'window period', which is two weeks for Chlamydia / Gonorrhoea and six weeks for Syphilis / HIV.  If low risk individuals are tested then the numbers of individuals to test to find one infection is high.  For example if 1% of the tested population has a single infection then 100 people would need to be tested to find one infection.  The National Chlamydia Screening Program tests young people less than 25 years of age because the prevalence of infection in this group is around 10%; consequently, for every ten young people tested one Chlamydia infection is found.

The picture below shows the analysis from Guy's & St Thomas' from the first six months of 2015 for Gonorrhoea.  This bacterium is important because it is very infectious and it evolves antibiotic resistance over time.  In the UK, Gonorrhoea is mostly diagnosed in men who have sex with men and in heterosexual from the Black African / Caribbean community. 

This data shows the importance of informing partners and getting them tested.  One in three partners need to be tested to find one Gonorrhoea infection.  There is no difference in the number of male & female partners that need to be tested to find one infection in this analysis even though there are 17 times more diagnoses of Gonorrhoea in men (1 in 16 versus 1 in 283).

IF all contactable partners of an STI are informed & tested then it would be theoretically possible to significantly reduce an infection from a sexual network; however, to date no single tool has been developed to support the person with the infection, partners and providers to effectively deliver partner notification and measure the impact of this important service.