How STI testing works
STI tests answer one of two questions. Antibody tests ask whether your immune system has responded to an infection. Direct tests ask whether the organism is present in the sample. That difference explains almost everything about timing.
Diagnik Health Library · Updated
Antibody tests
Used for HIV and syphilis. They are simple and quick, but they depend on your immune response, which takes time — hence the window period. Antibodies can also persist after a treated infection, which is why a reactive syphilis result needs clinical interpretation.
Tests that detect the organism
Used for chlamydia, gonorrhea and trichomoniasis. They look for the organism at the site sampled, so they become positive sooner — and they only tell you about the site you actually sampled.
Screening versus diagnosis
- A screening test sorts people into needs-follow-up and does-not.
- A diagnosis is a clinical conclusion combining tests, history and examination.
- An over-the-counter self-test is always the first of those two.
Tests mentioned in this guide
OraQuick In-Home HIV Test
An HIV answer at home, in 20 to 40 minutes, with no blood.
First To Know Syphilis Test Kit
A single drop of blood, a syphilis answer in 15 minutes.
Visby Women's Sexual Health Test
Three common infections from one self-collected swab, in about 30 minutes.
Frequently asked questions
- Why do some tests need a longer wait than others?
- Because antibody tests depend on your immune response developing, while tests that detect the organism can be positive as soon as it is present at the site sampled.
- Why is a reactive result called preliminary?
- Screening tests are tuned to miss as few true infections as possible, which produces a small number of reactive results that are not true infections. Confirmatory testing resolves them.
Not sure which test you need?
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Find my testKeep reading
- hiv
How HIV testing works
Antibody, antigen/antibody and nucleic acid tests, and where an over-the-counter self-test fits.
- syphilis
Syphilis testing
Screening finds antibodies; a clinician works out whether treatment is needed now.
- chlamydia
Chlamydia testing
Which sample is used, and why site and anatomy change the answer.
Related testing pages
Sources
Educational information only, not a medical diagnosis or treatment advice. Screening results always need confirmation by a clinician.
This guide is general health information about screening tests. It is not a diagnosis and does not replace care from a clinician. See our medical disclaimer.