Window period: when an HIV or syphilis test is reliable
A test taken soon after an exposure can come back negative even when there is an infection. Learn what the window period is, when to repeat the test and why PEP cannot wait.
The window period is the time between contact with a virus and the point at which a test can detect the infection. For HIV, Brazil's Ministério da Saúde (Ministry of Health) states that the body takes 30 to 60 days to produce detectable antibodies, so a negative result soon after an exposure may be false. The advice is to test now and repeat the test after at least 30 days. The SUS (Brazil's public health system) offers free tests for HIV, syphilis and hepatitis B and C. If the exposure happened less than 72 hours ago, seek in-person care immediately to be assessed for PEP.
Understanding the result
What the window period is and why it matters
Every test has a window period: a stretch of time when an infection may be present but does not yet show up on the test.
The Ministério da Saúde calls this interval the “diagnostic window”: the time between contact with the virus and detection of the infection marker, which may be an antigen or an antibody. If the test is done during this period, the result can be negative even though the person is infected.
For HIV, the Ministério da Saúde states that the immune system takes 30 to 60 days to produce detectable antibodies. The window varies with the type of test. The World Health Organization reports that, in most cases, antibodies appear within 28 days of infection, and that during this period many rapid tests cannot yet detect them.
Timeframes vary with the type and generation of test used. That is why the clinician or service that ordered the test decides whether the result is already conclusive, and when to repeat it, based on the date of exposure and the test performed.
Sources: Ministério da Saúde, diagnosis of STIs and viral hepatitis; Ministério da Saúde, window period; WHO, HIV and AIDS.
When not to wait
Recent exposure: PEP depends on the first few hours
If the exposure happened recently, the most urgent question is not about testing but about HIV post-exposure prophylaxis, known as PEP.
According to the Ministério da Saúde, PEP is indicated in situations such as sexual violence, sex without a condom or with a condom that broke, and occupational accidents involving sharps or direct contact with biological material. The decision on whether to use PEP is made at the health service, after the situation has been assessed.
If the 72-hour deadline has already passed, testing and follow-up are still important. Seeking help after an exposure, whatever the circumstances, is a way of looking after your health, not a reason to be judged.
Sources: Ministério da Saúde, PEP; Ministério da Saúde, window period.
Test and repeat
When to get tested and when to repeat the test
The Ministério da Saúde advises anyone who has been in a risk situation to get tested straight away and repeat the test after 30 days.
If the result is negative and an infection is still suspected, the test should be repeated after at least 30 days, because the first test may have been done within the window.
- Write down the exact date of the exposure; it guides the timing of each test.
- Keep your results together with the date they were done and the type of test.
A reactive result also needs follow-up: the WHO points out that no single test confirms an HIV diagnosis and that confirmation is made by trained health professionals.
Sources: Ministério da Saúde, window period; Ministério da Saúde, diagnosis of STIs and viral hepatitis; WHO, HIV and AIDS.
Reactive results
Reactive rapid syphilis test: what happens next
The rapid syphilis test detects antibodies against the bacterium Treponema pallidum and is free on the SUS, with a result within 30 minutes.
If the rapid test is reactive, the Ministério da Saúde states that the diagnosis is completed with a further blood test carried out in a laboratory. In some situations, such as pregnancy, sexual violence, signs of syphilis or a risk of being lost to follow-up, treatment may start on the basis of the rapid test, at the clinician's discretion.
Anyone who has had syphilis before should know that treponemal rapid tests tend to stay reactive even after treatment and cure. So a reactive result in someone who has already been treated must be interpreted alongside their history and the laboratory tests the clinician requests.
For hepatitis B and C, the SUS also offers free tests. The diagnostic window rule applies to all tests; the service that carries out the test will advise whether and when to repeat it.
Sources: Ministério da Saúde, syphilis; Ministério da Saúde, diagnosis of STIs and viral hepatitis.
Access
Where to get tested on the SUS and when a teleconsultation helps
Rapid tests use a drop of blood from a fingertip or a sample of oral fluid and give a result in 30 minutes at most.
You can go to a public primary care unit (unidade básica de saúde) or a Centro de Testagem e Aconselhamento (CTA, a testing and counselling centre). At a CTA, tests can be done anonymously, with counselling to help you understand the result. To find out where to get tested, you can also call Disque Saúde, the health helpline, on 136. There is also the HIV self-test, which people carry out and read on their own or with someone they trust.
Outside the PEP window and with no signs of an emergency, an online sexual health consultation can help you organise the dates, understand which tests make sense in your case and when to repeat them. The online consultation offers guidance but does not replace urgent in-person care for PEP, or a physical examination when one is needed.
Source: Ministério da Saúde, diagnosis of STIs and viral hepatitis.
Next step
Choose your care based on the time since the exposure
An exposure less than 72 hours ago needs immediate in-person care. After that, a clinician can help you plan your tests and repeat tests.
- Online sexual health and STI consultation
- Doctors available in Brazil
- How requesting tests works
- Contact the team
Bring your dates to the consultation
Write down the date of the exposure, the tests you have already had and their results. If the exposure was less than 72 hours ago, go to an in-person service first.
References
Medical sources consulted
Official sources checked on 5 October 2026.
This article summarises the public guidance from the Ministério da Saúde and the WHO on the window period, free testing on the SUS and the PEP deadline. It does not give specific window periods for each test generation or for hepatitis, does not mention medicines or doses, and does not replace assessment by whoever ordered or performed the test.
Questions
Frequently asked questions
I took a test a week later and it was negative. Can I stop worrying?
Not yet. A test taken within the window can be negative even when there is an infection. The Ministério da Saúde advises repeating the test after at least 30 days if an infection is still suspected.
More than 72 hours have passed. Is it still worth seeking care?
Yes. The deadline for starting PEP has passed, but testing, repeating the test on the right date and getting follow-up are still important. The SUS offers the tests free of charge.
Can an online consultation replace PEP?
No. PEP requires in-person care at a health service as soon as possible. An online consultation helps you plan tests and repeat tests when it is not urgent.
