When to test for STIs after sex

Sexual assault, significant genital injury, pregnancy risk, known exposure to an STI or severe symptoms also warrant prompt clinical assessment rather than waiting for a routine test date.

The 72-hour limit is the single most consequential piece of timing in sexual health, and it is frequently missed because people assume the correct response to a worrying exposure is to book a test. If HIV exposure is a realistic possibility, the priority is being assessed now — sooner is better within that window, and hours matter.

The window period is the interval between acquiring an infection and when a particular test can reliably detect it. Different organisms and test technologies have different windows.

This is why “I got tested” is an incomplete statement. Tested for what, using which method, and how long after which exposure? Two people can both have a negative result and be in entirely different positions.

Exact timing should be interpreted with the laboratory method used, because a point-of-care test and a laboratory assay may not share the same window.

One consequence worth stating plainly: taking HIV PEP changes the follow-up schedule. Testing while on or shortly after PEP requires a specific plan, because the medication affects when infection would become detectable. That schedule is set by the prescribing clinician, not by a standard screening interval.

Chlamydia and gonorrhoea can infect the throat and rectum as well as the genital tract, and a urine sample does not detect infection at those sites. Someone who has had oral or anal sex and tests urine only can receive a clear result while carrying an untreated infection.

This is probably the commonest way people are wrongly reassured. Throat and rectal infections are frequently asymptomatic, so nothing prompts further testing, and they remain transmissible.

Consultations here are confidential, and the purpose of these questions is entirely practical.

Packages advertised as comprehensive vary substantially, and the differences are rarely obvious from the name. Before booking, it is reasonable to ask exactly which infections are covered and which samples are taken.

Herpes deserves a note. Routine blood testing for herpes in people without symptoms is generally not recommended, because the tests can be difficult to interpret, may not distinguish site of infection, and a result can cause considerable distress without changing management. Testing is different when there are symptoms or lesions to sample.

Unusual discharge, painful urination, genital ulcers, pelvic or testicular pain, rash, fever or swollen lymph nodes deserve assessment. The clinician may test immediately and repeat later if necessary.

Many STIs are asymptomatic, so absence of symptoms does not remove the need for testing when exposure risk is significant.

An early negative test can be genuinely useful while still being too early to close the case. Repeat testing may be advised when:

• The first test was performed inside the expected window period.
• There was a known positive partner.
• Symptoms develop after an initial negative test.
• HIV PEP or PrEP changes the recommended follow-up schedule.
• A diagnosed chlamydia or gonorrhoea infection requires later retesting because reinfection is common.

That last point is often misunderstood as a test of whether treatment worked. It is usually about reinfection — commonly from an untreated partner — which is why partner treatment and retesting are handled together.

If an infection is diagnosed, recent partners need testing and often treatment. Without it, reinfection is likely and the infection continues to spread to people who have no reason to suspect it.

• Clinics can advise on which partners and what time period to cover.
• Notification can often be done anonymously if preferred.
• Avoid sex until treatment is complete and any advised interval has passed.
• Being treated does not prevent reacquiring the same infection.

This conversation is uncomfortable for most people. It is also the single action that most reduces onward transmission, and clinics are well practised at helping with the wording.

• Use condoms appropriately for vaginal and anal sex.
• Discuss PrEP if ongoing HIV exposure risk is significant.
• Know that PEP is an emergency option after selected recent HIV exposures.
• Vaccination can prevent hepatitis B and HPV-related disease.
• Test at appropriate intervals when you have new or multiple partners or other risk factors.

A sexual-health history helps tailor testing to the actual exposure sites, including throat or rectal swabs when relevant.

The distinction between PrEP and PEP is worth being clear about: PrEP is taken in advance by people with ongoing risk, while PEP is an emergency course started within 72 hours of a specific exposure. They are not interchangeable, and PEP is not a substitute for ongoing prevention where risk is recurring.

Condoms also reduce but do not eliminate transmission of infections spread by skin contact, such as herpes and HPV, since they do not cover all potentially affected skin. That is a reason for vaccination and testing rather than a reason to skip condoms.