FAQs

This question usually refers to how soon after exposure can someone test for HIV.

This usually requires waiting four weeks before taking an antibody-based test.

The latest UK guidelines state that 4th generation HIV tests (antigen/antibody) will detect 95% of infections four weeks after exposure.

A negative test after four weeks then needs to be confirmed with a second test three months after the risk. This is because some people take longer than four weeks to generate an antibody response.

In high risk exposures, especially if symptoms occur, viral load testing is sometimes used after one week.

This includes after a sexual assault or after a needlestick injury to a health care worker.

In these cases a viral load test can exclude an infection when there are symptoms.

Viral load tests are not approved to diagnose HIV. A negative result still needs to be confirmed by an antibody test three months after the risk.

Recommended time from exposure to HIV test

The window period is time between potential exposure to HIV infection and the point when the test will give an accurate result.

During the window period a person can be infected with HIV and infectious but have a negative HIV test.

The window period for a 4th generation antigen/antibody test is four weeks. At this time this test will detect 95% of infections. After a three month window period after exposure, the confirmatory test will detect more than 99.97% of infections.

The earliest marker is HIV viral load. This is in the first weeks after infection (usually from 1 to 6 weeks after exposure). A high viral load is related to seroconversion symptoms. The first HIV protein (antigen) that can be measured is p24 (from 1 to 8 weeks after exposure).

Viral load and p24 tests are not accurate for diagnosing early HIV if the results are negative.

An HIV antibody response can be detected as early as two weeks in a few people and in more than 99.9% of people by 12 weeks. An antibody test at 4 weeks will detect 95% of infections.

Antibody testing at 4 weeks can give you a good indication of your HIV status, but you need a test at 12 weeks after the exposure to be considered HIV negative.

Before taking an HIV test someone at the clinic should explain what is involved. This should include information about the type of test and test accuracy.

It should include information about what happens if the results are positive. It is important that you know about what happens if the results are positive.

Blood samples can be from a pinprick or having blood taken into a test tube.

Oral tests involve rubbing a swab on your gums to collect samples of cells.

Your test centre should clearly explain the results of your test.

If you have questions that were not explained, or that still worry you, ask the test centre first.

Rapid blood tests show two lines if positive or one line if negative, in a similar way as a pregnancy test (see Figure 1).

Results from laboratory tests are given as negative, positive or indeterminate.

• Negative or non-reactive means you are HIV negative. You do not have HIV (based on the window period and no recent risks).
• Positive or reactive means the test shows you are HIV positive and you have HIV infection.
• Indeterminate means the test result was unclear and needs to be repeated.

Figure 1: Example results from a rapid test

Yes. Modern HIV tests are very accurate.

This accuracy has to be considered with the window period.

For example, 4th generation tests will pick up 95% of infections at 28 days after exposure.

A confirmatory test three months after the exposure is always recommended. This is because 5% of people take this long to show a positive result.

A positive test result is routinely confirmed using a different type of test called western blot. The western blot test looks for immune responses to specific HIV proteins.

When used as a confirmatory test, the western blot result is 100% accurate. (This assumes that there is not a lab error and that it is your sample that has been tested).

HIV antibody tests are not affected by other circumstances.

This includes infections, medications, most vaccinations, putting on weight, eating or drinking anything before the test, use of alcohol or recreational drugs, mouthwash or time of day.

Your test result is accurate even if you had flu or a cold or are using any medication.

You do not need to fast before your test. Food and drink do not affect the results.

This will depend on how recent your last exposure was.

As part of good practice, if the exposure was less than three months ago then testing at three months after exposure is usually recommended.

HIV tests after the 3 month window are more than 99.97% accurate. They work for all types and subtypes of HIV.

Very few medical tests have 100% accuracy. There will still be rare cases where someone is HIV positive and not picked up.

However, HIV tests are one of the more accurate tests for any medical infection. Tests showing a negative result are ‘interpreted’ as negative.

If the result is negative three months after exposure your result is interpreted as negative. This assumes you have had no further risks.

At this point you can stop worrying. This is the purpose of testing. Learn from the experience you have gone through in taking a test.

Learn about how to protect yourself in the future so you don’t have to go through this stress again. This will enable you to make informed decisions and to look after your sexual health.

If the result is negative four weeks after the exposure, this tells you that you are very likely to be HIV negative. The test after three months will confirm this.

A false negative test result occurs when the test shows negative and the person is really HIV positive.

This is very rare and usually occurs during the window period when people are newly infected but the test can’t quite pick up the infection.

As with other types of tests, there is always be a small margin of error. With antibody-only tests (3rd generation) only 0.3% of tests (3 tests in every 1000) will be a false negative after 3 months.

With 4th generation tests this is even lower. In practice, a negative result after three months means you do not have HIV.

You do not need to test again unless you expose yourself further in the future.