Investigations for STIs

Along with clinical review, microbiological testing is integral to the diagnosis of sexually transmissible infections (STIs). Nucleic acid amplification testing (NAAT) (eg polymerase chain reaction [PCR]) is the preferred test for the diagnosis of many STIs because it is highly sensitive. NAAT can occasionally produce false-positive results. This is usually due to cross-contamination, or if the test is repeated too soon after treatment because NAAT does not differentiate between living and dead organisms. Therefore, confirm an unexpected positive result with another sample or testing method, particularly when a positive result has serious implications.

Concurrent STIs are not uncommon, particularly in populations with a higher prevalence of STIs, such as gay, bisexual and other men who have sex with men, and transgender womenCallander, 2019Ong, 2023. Concurrent STIs are often asymptomatic infections of the rectum or pharynx. For patients with a suspected STI, consider testing for Chlamydia trachomatis, Neisseria gonorrhoeae1, HIV and syphilis, and for patients who are not fully vaccinated, consider testing for hepatitis A and B. Immune compromise due to HIV infection often leads to severe cases of genital herpes, warts and molluscum contagiosum; test for HIV in these situations. Seroconversion for HIV and syphilis may not occur for up to 6 weeks after exposure.

For additional information to aid diagnosis of STIs, see:

1 Because of increasing rates of antimicrobial resistance, cultures should always be requested for known or suspected gonorrhoea prior to treatment.Return