HIV (human immunodeficiency virus)

HIV infection is usually acquired through sexual intercourse, exposure to infected blood, or perinatal transmission.

Persons with high-risk exposure should be referred for prophylactic antiretroviral treatment immediately, prior to laboratory testing.

Sonic laboratories currently use a screening immunoassay that detects both HIV p24 antigen and antibodies to HIV-1 and HIV-2. The inclusion of p24 antigen in the screening test decreases the ‘window’ period between infection and seroconversion. An HIV viral load is not recommended for diagnosing HIV infection prior to seroconversion, as false positive results can occur. Positive antigen/antibody screening tests may begin to develop as early as 10 days post exposure, and additional antibodies appear on the Western blot over the following two weeks. By six weeks after exposure, results are reliable but legally, a negative result cannot be reported until repeat testing, three months after the initial test.

If a person is being tested because of a particular low-risk exposure, testing is recommended at baseline and three months.

Any results with reactivity in the screening test are automatically forwarded for supplementary testing to a state HIV reference laboratory. The additional test is an HIV Western Blot. This detects antibodies against several HIV antigens and a certain number of antibodies must be present before a Western Blot is reported as positive. Early and late in infection the Western blot may show an indeterminate profile.

The reference laboratory also performs HIV p24 antigen testing on the blood sample.

Low-level reactivity in the screening test may occur early in seroconversion or be due to non-specific (false-positive) reactivity. Non-specific results, usually to a single viral antigen, occur in approximately 1:500–1:1,000 low-risk pregnant women. In the latter case, the Western blot does not develop any further antibodies over time, and after a three-monthly follow-up period, the Western blot will be reported as unchanged and not indicative of HIV infection.

A new positive HIV diagnosis will be phoned to the ordering doctor by the microbiologist.

It is always recommended that a new diagnosis be confirmed by a repeat blood test on the patient.

Counselling advice and referral advice is available from various local agencies, often most useful prior to seeing the patient and giving the diagnosis. The microbiologist can advise you of local contact details. See also the Australasian Society for HIV Medicine, link below.

Baseline tests may be ordered when repeating the initial positive HIV testing, including:

See Occupational blood and body fluid exposure; CD4 T-lymphocytes (T-helper cells)