In short
- Every infection has its own window period — the time before a test can detect it. A negative result taken too early means nothing.
- HIV with a modern fourth-generation test is reliable six weeks after a risky contact, and conclusive at three months.
- Chlamydia and gonorrhea are detected with a molecular test, not a blood test: first-catch urine or a swab.
- If the contact was oral or anal, urine is not enough. A swab of that site is needed.
- Most of these infections cause no symptoms. Having no complaints is not proof that there is no infection.
With sexually transmitted infections the most common mistake is testing too early. A negative result taken three days after a risky contact tells you nothing — only that it was too soon.
The window period: why timing is key
The window period is the time between infection and the moment a test can detect it. During this time the result can be negative even though the infection is present. The person can already be infectious. Window periods differ between infections:
| Infection | When the test is reliable | When it is conclusive |
|---|---|---|
| HIV (fourth-generation test, antigen and antibodies) | 6 weeks | 3 months |
| Chlamydia (molecular test) | 2 weeks | — |
| Gonorrhea (molecular test) | 2 weeks | — |
| Syphilis (antibodies) | 6 weeks | 3 months |
| Hepatitis B (HBsAg) | 6 weeks | 3 months |
| Hepatitis C (antibodies) | 8–12 weeks | 6 months |
If the contact was less than two weeks ago, wait. The only exception is if you have symptoms. Then get tested right away, because the cause needs to be found and treated.
If the contact was very high-risk and less than 72 hours have passed, post-exposure prophylaxis for HIV exists. It must be started as soon as possible. This is not a laboratory question. Go to an emergency department or an infectious diseases clinic immediately.
Which sample is right
The wrong sample is the second most common reason for a falsely reassuring result. Chlamydia and gonorrhea are not detected in blood.
| Infection | Sample | Note |
|---|---|---|
| HIV, syphilis, hepatitis B and C | Blood. | No fasting needed. |
| Chlamydia and gonorrhea in men | First-catch urine. | Do not urinate for at least one hour before the sample. |
| Chlamydia and gonorrhea in women | A vaginal swab, which you can take yourself. | As reliable as a swab taken by a doctor, and more reliable than urine. |
| After oral contact | A throat swab. | Urine does not detect these infections. The throat often does not hurt. |
| After anal contact | A rectal swab. | Also without symptoms in most cases. |
When testing makes sense
- With a new partner or after the end of a relationship, before starting a new one.
- After unprotected contact or if protection broke.
- With symptoms: burning when urinating, discharge, skin changes, sores, pain in the lower abdomen, pain in the testicles.
- If a partner has told you about an infection. Get tested even without symptoms.
- Before a planned pregnancy, both partners.
- Regularly, if you have several partners — the frequency is adjusted to the risk.
People often overlook this: chlamydia in women causes no symptoms in the vast majority of cases. But left untreated, it can damage the reproductive organs. That is why testing does not depend on whether anything hurts.
How testing works
- Where to test: a sexual health clinic, your GP or a laboratory that offers STI testing.
- The result is yours — clinics and laboratories are bound by medical confidentiality.
- You can usually collect urine and swab samples yourself, in private.
- A result with an explanation — already have a report? Upload it to Blood Lab 360 and get every value explained.
If the result shows an infection that needs treatment, the next step is a doctor. A doctor prescribes the medicines for these infections. You cannot buy them on the basis of a result.
After the result
| Result | What it means | What to do |
|---|---|---|
| Negative after the window period | No infection. | Nothing. Repeat after a new risk. |
| Negative before the window period has passed | Tells you nothing. | Repeat after the window period for that infection. |
| Positive | An infection that needs treatment. | See a doctor. No sexual contact until treatment is finished. |
| Borderline or unclear | Confirmation with another method is needed. | A confirmatory test; do not draw conclusions from the first result. |
With a confirmed infection it is important to inform recent partners so they can get tested. It is not pleasant, but it is the only way to break the chain. With chlamydia it is often the only way a partner finds out at all.
Frequently asked questions
When after a risky contact can I test for HIV?
With a modern fourth-generation test the result is reliable six weeks after the contact, and conclusive at three months. A negative result taken earlier tells you nothing.
Is chlamydia detected in blood?
No. Chlamydia and gonorrhea are detected with a molecular test on first-catch urine or a swab. Blood is not the right sample for these two infections.
I have no symptoms. Is it still worth testing?
Yes. Chlamydia in women causes no symptoms in the vast majority of cases. Gonorrhea in the throat often causes none either. Having no complaints is not proof that there is no infection.
Is testing anonymous?
That depends on where you test — many sexual health clinics offer anonymous or confidential testing. If the result is positive you will need a doctor, because the medicines are prescription-only.
What should I do if the contact was less than 72 hours ago and the risk was high?
Do not wait for the laboratory. Post-exposure prophylaxis for HIV exists and must be started as soon as possible. Go to an emergency department or an infectious diseases clinic immediately.
Sources: ECDC — window periods and recommendations for STI testing · WHO — screening and management of sexually transmitted infections · BASHH — sample choice, self-collected swabs and extragenital sampling sites · IUSTI Europe — European guidelines on the management of chlamydia and gonorrhea · NIJZ (Slovenian National Institute of Public Health) — HIV post-exposure prophylaxis.
