An HIV diagnosis in Australia is based on detecting antibodies to HIV in a blood sample. Combination HIV antibody/antigen testing is initially undertaken as a screening test for HIV, detecting the presence of either antigen or antibody. Positive screening tests must be confirmed by a diagnostic assay such as a Western Blot assay. If the requesting clinician is uncertain as to how soon after infection a given test will yield a positive result, a window period of two to six weeks should be used.
HIV testing can also be performed using rapid point-of-care testing which provides results within 30 minutes. Point-of-care tests have a longer window period than laboratory-based assays. Any positive point-of-care test must be followed up with definitive diagnostic testing using laboratory-based assays.
Window period
There is a period after infection, when the screening test will not detect either antibody or antigen because they are yet to be produced or are present at a level that cannot be detected. This is called the window period. The time after infection at which antibody is identified is a function of the individual's response to HIV and the type of test used, but it is usually in the order of two to six weeks. Although HIV testing may be negative, the person is usually highly infectious.
- A negative screening test for HIV excludes HIV infection provided that the last potential HIV exposure was at least 12 weeks before the test. If not, the test must be repeated at an appropriate time.
- Some reactive EIA results are not due to HIV infection. Therefore, all repeatedly reactive results must have confirmatory tests performed such as an HIV Western Blot test and/or a nucleic acid or p24 antigen test if acute HIV infection is suspected.
- If the Western Blot test is positive, then HIV infection is highly likely. It is recommended that the HIV antibody tests be repeated on a second blood specimen from the patient to confirm the diagnosis of HIV infection.
- Tests for detection of HIV proviral DNA are available but are only used in special circumstances, for example when there is uncertainty about the diagnosis of primary HIV infection or for the assessment of the babies of HIV-infected women. This should be discussed with a specialist in HIV medicine or infectious diseases (see list of contacts in contacts for specialist advice on STIs and HIV).
Acute HIV infection presents with non-specific symptoms after which the infection is asymptomatic for years so opportunistic testing is important.
For information about who and how to test, see Decision Making in HIV | ASHM (external site).
If a patient presents with a positive self-test, ensure it is confirmed with a laboratory test.
People who should be offered testing include:
- Populations with higher prevalence of HIV:
- people who have or are being investigated for any STI or BBV
- people presenting with symptoms consistent with HIV primary infection and/or unusual or persistent infections for which there is no adequate alternative explanation
- people born in intermediate and high prevalence countries
- people who have engaged in risk behaviour/exposure in high prevalence countries
- Aboriginal and Torres Strait Islander peoples
- sexual and injecting partners of people living with HIV
- people who have ever injected drugs
- men who have sex with men
- transgender women and people who identify as gender diverse who have sex with men
- people who have recently changed partners, who have multiple concurrent sex partners
- people in custodial settings or who have been incarcerated in a custodial setting
- people who have received healthcare, including blood transfusion, overseas where there may be poor infection control practices.
- Populations with higher risk of onward transmission and/or adverse health outcomes:
- pregnant people
- infants and children born to mothers who have HIV
- people initiating HIV pre-exposure prophylaxis (PrEP)
- health-care workers conducting exposure-prone procedures.