Anti-HIV 1, 2, p24 Antigen Test

The Anti-HIV 1, 2, p24 Antigen Test is a fourth-generation combination test: it simultaneously checks blood for antibodies to HIV-1 and HIV-2 and for p24 antigen, a protein of the virus itself. The antigen enables earlier detection than older antibody-only tests. It is performed after a potential exposure, as part of sexually transmitted infection testing or when a doctor suspects HIV.

Who the test is for

• people who have had unprotected sex with someone whose HIV status they do not know, experienced a condom break or shared injecting equipment;
• people who want to stop using condoms with a new partner;
• people diagnosed with another sexually transmitted infection or hepatitis B or C;
• women planning pregnancy and pregnant women;
• people who develop fever, a sore throat, rash or swollen lymph nodes several weeks after a potential exposure, which can accompany recent infection.

What the test measures

HIV (human immunodeficiency virus) damages the immune system and, without treatment, leads to AIDS after a number of years. It is transmitted through unprotected sex, blood, for example shared needles, and from mother to child during pregnancy, birth and breastfeeding. It is not transmitted through handshakes, hugs, shared dishes or mosquito bites.

The p24 antigen appears in blood as early as two to three weeks after infection, before antibodies. Once the body produces antibodies, p24 falls, so antigen alone is not sufficient to demonstrate infection. HIV antibodies remain in blood for life. The test also covers HIV-2, which is rare in Europe.

If fewer than 72 hours have passed since a potential exposure, do not wait for a test. Contact an infectious disease specialist or emergency department immediately about post-exposure prophylaxis (PEP), medicines that can prevent infection if started in time.

The window period: when results are reliable

The window period is the time from infection until a test can detect it. A combination laboratory test using venous blood usually detects HIV 18 to 45 days after exposure. Under British testing guidelines (BHIVA, BASHH and BIA, 2020.), a negative fourth-generation test is considered conclusive if at least 45 days have passed since the last potential exposure.

If you test earlier, for example after four weeks, a negative result is reassuring but should be repeated once 45 days have passed. If you took PEP or PrEP after exposure, a doctor will determine when to retest. Statements in older texts that antibodies develop 4 to 12 weeks after infection refer to older tests.

Preparation and sample

The test uses blood taken from a vein. No special preparation or fasting is needed.

How results are interpreted

A negative (non-reactive) result means no antibodies or antigen were found in the sample. If at least 45 days have passed since the last potential exposure, there have been no new exposures and you have not taken PEP or PrEP, a negative fourth-generation laboratory test using venous blood is considered conclusive under the above guidelines. After PEP or PrEP, a doctor sets the testing schedule.

A reactive (positive) result is not a final diagnosis. Every reactive result is confirmed using another method because false positives are possible, although rare. Do not draw conclusions before confirmation; a doctor, usually an infectious disease specialist, guides further steps. If infection is confirmed, modern treatment allows a long, good-quality life, and a person whose viral load is undetectable with treatment does not transmit HIV sexually.

A PCR HIV detection test is also available to detect viral genetic material. It does not replace the combination test and is used in particular circumstances on medical recommendation.

Privacy and counselling

You can also be tested anonymously with us, without having to disclose personal information. If you test under your name, the test details and results are confidential medical information, kept in accordance with medical confidentiality and personal data protection rules. If you would like to discuss risk and testing with a professional, Croatia also has centres for voluntary, anonymous, free HIV counselling and testing, coordinated by the Croatian Institute of Public Health.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed on the Contact page), and anonymous testing is also possible. Prices for all tests and panels are available in the service price list. If you also want testing for other sexually transmitted infections, the tests and panels are described on the Sexually transmitted infection testingpage.

Frequently asked questions

How long after a potential exposure can I take an anti-HIV test?

You can test a few weeks after exposure because the combination test detects many infections by then. However, a negative result is considered conclusive only once at least 45 days have passed since the last potential exposure, with no new exposures and provided you have not taken PEP or PrEP. After PEP or PrEP, a doctor sets the testing schedule. Within 72 hours of exposure, seeking medical care for PEP is more important.

What does a reactive anti-HIV result mean?

A reactive result means the test reacted to antibodies or antigen, but does not confirm infection on its own. Confirmation using another method is needed, after which a doctor gives the final interpretation.

Can I be tested anonymously?

Yes. Anonymous testing is available at our branches without having to disclose personal information. Anonymous, free testing with counselling is also offered by centres coordinated by the Croatian Institute of Public Health.

Should I test for other sexually transmitted infections too?

After a potential exposure, it is often useful to check for syphilis and hepatitis B and C, which spread through similar routes. The tests needed depend on the type of contact and symptoms, so it is best to discuss them with a doctor.