HIV test
Also known as: HIV antibody test, fourth-generation HIV test, HIV screening test, HIV blood test
A blood test that looks for the human immunodeficiency virus, usually by detecting both a viral protein (p24 antigen) and the antibodies the immune system makes against the virus. Modern combined tests become positive within a few weeks of infection; a positive screening result is always confirmed with a second test before a diagnosis is given.
- Canonical name
- HIV antigen and antibody test
- Abbreviations
- HIV Ag/Ab
- Kind
- Laboratory test
- Categories
- Infectious disease & microbiology, Point-of-care & home testing
- Specimen
- Serum, Capillary (fingerprick) blood, Saliva
- Method
- Immunoassay, Point-of-care device, PCR (including RT-PCR and qPCR)
- Terminology
- LOINC 56888-1
- Type
- Blood test (serology; rapid and self-tests available)
- Sample
- Blood from a vein, a fingerprick, or saliva for self-tests
- Used for
- Screening for HIV, testing after exposure, monitoring treatment once diagnosed
- Measures
- HIV antibodies and p24 antigen; viral load and CD4 count after diagnosis
- Result
- Negative, reactive or confirmed positive; numeric viral load and CD4 count in monitoring
Why is it used?
- Routine screening: HIV testing is offered widely because early treatment protects health and prevents transmission
- After a possible exposure, at the right interval
- Symptoms of seroconversion or of an immune system under attack
- Pregnancy screening, before some treatments and in people diagnosed with other sexually transmitted infections
What does it measure?
| Measure | What it tells you |
|---|---|
| HIV-1 and HIV-2 antibodies with p24 antigen | The standard fourth-generation laboratory screen, reliable from about 45 days after exposure; antigen detection shortens the window. |
| Rapid and self-tests | Fingerprick or saliva tests with results in minutes; a reactive result must be confirmed by a laboratory. |
| HIV RNA (viral load) and CD4 count | After diagnosis: the amount of virus and the number of CD4 immune cells, used to plan and monitor treatment rather than to screen. |
A high or low value can be associated with several different situations; the result is interpreted with other results, symptoms, history and clinical findings.
How is it performed?
A venous blood sample in a laboratory, a fingerprick rapid test in a clinic or pharmacy, or a home self-test kit.
Specimen. Venous blood (serum) in the laboratory; capillary blood or oral fluid for rapid and self-tests
Do I need to prepare?
None. The window period matters: a test taken too soon after exposure can be negative, so a repeat at the recommended interval may be advised.
What do the results generally mean?
| Finding | What it may indicate |
|---|---|
| Negative after the window period | HIV infection is very unlikely; repeat testing is advised after any further exposure |
| Reactive screening test | Possible infection; a confirmatory laboratory test on a new sample is done before any diagnosis |
| Confirmed positive | HIV infection; referral to specialist care, where viral load and CD4 count guide antiretroviral treatment that keeps the virus suppressed |
| Negative inside the window period | Inconclusive; the test is repeated at the interval recommended for the type of test used |
What can affect the result?
- The interval since exposure (window period), which depends on the test type
- Post-exposure or pre-exposure prophylaxis medicines, which can delay antibody development
- Rare false-reactive screening results, which is why confirmation is required
Medicines. Antiretroviral prophylaxis can delay or blunt antibody responses; antiretroviral treatment lowers the viral load, which the monitoring tests track.
What can the test not tell you?
- Whether infection occurred within the last few weeks, before markers appear
- The stage of infection or the state of the immune system, which need the viral load and CD4 count
- Other sexually transmitted infections, which have their own tests
No test detects infection immediately after exposure; results are only interpreted in relation to the timing of exposure, and a screening result is never a diagnosis on its own. Self-tests carry the same window-period limits and need confirmation.
Other tests commonly used with it
Catalogued concepts this page covers
Concepts of the master test taxonomy that resolve to this page; each links to its category.