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Sexual health testing guide · 2026
HIV testing is straightforward once you know which test you had and when it was taken relative to the exposure.
A fourth-generation HIV test detects both HIV antibodies and p24 antigen, which shortens the window compared with antibody-only tests. It detects most infections from around 18 to 45 days after exposure, is highly reliable at 45 days, and is considered definitive at 90 days. Rapid point-of-care tests are mostly antibody-based with a longer window, and any reactive result requires laboratory confirmation. HIV RNA (NAT) testing has the shortest window but is not a routine screening test. If the exposure was within 72 hours, post-exposure prophylaxis matters more than any test.
| 4th-generation test | Antibody plus p24 antigen; detects most infections from around 18–45 days |
|---|---|
| Definitive | 90 days after exposure; schedule restarts after a PEP course |
| Rapid tests | Mostly antibody-based, longer window; reactive results need confirmation |
| RNA/NAT | Shortest window (roughly 10–33 days) but not routine screening |
| Within 72 hours | PEP is time-critical and takes priority over testing |
| Price | Listed on the MedEx STD and STI testing page and the lab test catalogue |
| Fourth-generation laboratory test | Detects HIV antibodies and p24 antigen. The standard screening test in most settings. Reactive results go on to a confirmatory algorithm. |
|---|---|
| Rapid point-of-care test | Result in minutes. Mostly antibody-based, so the window is longer. Excellent for accessibility; any reactive result requires laboratory confirmation before diagnosis. |
| HIV RNA / nucleic acid test | Detects the virus itself, with the shortest window. Used for very early diagnosis in specific circumstances, for diagnosing infants, and for monitoring viral load. Not a routine screening test, and it can still miss very recent infection. |
When you receive a negative result, check which test it was and how long after the exposure it was taken. Those two facts determine what the result means. Available options are listed on the MedEx STI testing page and the lab catalogue.
Exposure within 72 hours? PEP takes priority over testing.
The window is the interval between infection and detectability. It exists because the test needs either enough viral protein or an antibody response to develop.
For a fourth-generation test, most infections are detected by around 18 to 45 days. A test at 45 days is highly reliable, and 90 days is treated as definitive in most guidance. Rapid antibody tests sit further out.
Two situations reset the clock. If you took PEP, testing restarts after the 28-day course finishes, because the medication can delay seroconversion. If you have ongoing exposure, each new exposure starts its own window, which is an argument for PrEP rather than repeated testing cycles.
The full comparison across infections is in the window periods guide.
Around half to two-thirds of people develop symptoms during early HIV infection, typically two to four weeks after exposure. The picture resembles glandular fever or a bad flu: fever, sore throat, swollen lymph nodes, a rash, muscle aches, mouth ulcers, headache and fatigue.
It is easily mistaken for dengue, influenza or a routine viral illness — and in Thailand, dengue is a common competing explanation. If you have these symptoms following a possible exposure, say so explicitly when seeking care, because it changes which tests are appropriate: an antibody test may still be negative while viral load is extremely high.
Recognising early infection matters. Viral load is highest at this stage, so transmission risk to others is greatest, and starting treatment early gives the best long-term outcome. Contact the 24/7 doctor on call rather than waiting.
Negative, past the window, no ongoing risk: no further action beyond routine screening appropriate to your circumstances.
Negative, but ongoing risk: this is the point to discuss PrEP — pre-exposure prophylaxis, which is highly effective at preventing HIV acquisition when taken as prescribed. It requires baseline testing, kidney function checks and regular follow-up. See MedEx PEP and PrEP services.
Reactive result: confirmation follows a defined laboratory algorithm. A single reactive screening test is not a diagnosis. If confirmed, modern antiretroviral treatment is highly effective, and someone on effective treatment with a sustained undetectable viral load does not transmit HIV sexually. Care should start promptly — arrange through a doctor consultation or specialist teleconsultation.
Test for other infections at the same time. Co-infection with syphilis, chlamydia, gonorrhoea and hepatitis is common, and hepatitis B vaccination should be checked — see travel and routine vaccination.
A fourth-generation test detects most infections from around 18 to 45 days, is highly reliable at 45 days and definitive at 90 days. If you took PEP, the schedule restarts after the 28-day course finishes.
A laboratory test detecting both HIV antibodies and p24 antigen. Including the antigen shortens the window compared with antibody-only tests, which is why it is the standard screening test in most settings.
They are valuable for accessibility, but most are antibody-based with a longer window than a laboratory fourth-generation test, so a rapid negative early in the window is less reassuring. Any reactive result requires laboratory confirmation.
Around half to two-thirds of people develop a glandular-fever-like illness two to four weeks after exposure, with fever, sore throat, swollen lymph nodes, rash, muscle aches and fatigue. It is easily mistaken for dengue or influenza, so mention any possible exposure when seeking care.
PEP is a 28-day emergency course started within 72 hours after a possible exposure. PrEP is taken before and during periods of ongoing risk and is highly effective at preventing acquisition when taken as prescribed.
No. A single reactive screening test is not a diagnosis and must be confirmed through a defined laboratory algorithm before any conclusion is drawn.
Ongoing risk? PrEP is more effective than repeated testing cycles.
Medical disclaimer: This article is general health information and does not replace medical advice, diagnosis or treatment. Laboratory reference ranges differ between laboratories and results must be interpreted alongside your symptoms, medicines and medical history. Speak with a qualified clinician before starting, stopping or changing any treatment. Service details, inclusions and prices can change — confirm them with MedEx before booking.
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