Window periods, infection by infection
This is the piece of information almost nobody has, and it makes half of all tests pointless. The clock starts at the date of the risky encounter, not the date you started worrying.
| Infection | Time before a reliable result | Sample |
|---|---|---|
| HIV — combined laboratory test | 6 weeks | blood sample |
| HIV — self-test or rapid test | 12 weeks | finger-prick |
| Chlamydia | 2 weeks | urine, or self-taken vaginal, throat or rectal swab |
| Gonorrhoea | 2 weeks | urine, or self-taken vaginal, throat or rectal swab |
| Syphilis | 4 to 6 weeks, recheck at 3 months | blood sample |
| Hepatitis B | 6 weeks to 3 months | blood sample |
| Hepatitis C | 6 weeks to 3 months | blood sample |
| Trichomoniasis, mycoplasma | when symptoms appear | local swab |
| Genital herpes | on a lesion | swab of the lesion |
| HPV | cervical screening by age, varies by country | cervical sample |
Why ask for a screen rather than a test
The commonest request at a laboratory is “an HIV test”. It is almost always the wrong request. In Western Europe HIV is the least likely of the six; chlamydia is the commonest, and the most silent. A full screen — HIV, syphilis, chlamydia, gonorrhoea, hepatitis B and C — costs barely more, often nothing at all, and answers the real question.
A word on samples: chlamydia and gonorrhoea also live in the throat and the rectum, and a urine sample alone misses them. If you have had oral or anal sex, say so: that is not a confidence, it is a technical instruction.
What can be prevented without a test
Two of these infections have a vaccine: hepatitis B and HPV. The second prevents cancers of the cervix, the anus and the throat, and can be caught up on in adulthood in most countries. HIV also has a preventive treatment, PrEP, for people at high exposure, and an emergency treatment after exposure, effective within 48 to 72 hours.