Dr Andrew Carr
Department of Immunopathology

1.                      Introduction

HIV is a notifiable disease with responsibility for reporting to Health departments resting with the testing laboratories. As such, there are standardised definitions, terminology and diagnostic algorithms relating to HIV diagnosis.

2.                      Diagnosis

A reactive antibody screening test result by ELISA does not mean that an individual has HIV-infection. Because positive screening antibody tests may not represent true biological positives, the terms reactive and non-reactive are used rather than positive or negative.

Diagnosis of HIV infection requires repeated reactivity on two or more HIV antibody tests, both repeated in duplicate, as well as a positive antibody status on a supplemental test, typically a Western blot, in which 4 or more reactive bands are detected. False-positive ELISA reactivity will generally have negative or atypical Western blot results (an exception is in infants born to HIV-infected mothers; the infants will have positive serology for 6 months or more reflecting passive transfer of maternal IgG). These false-positive results will initially be reported as indeterminate pending repeat testing.

A non-reactive antibody screening test result means that HIV-1-specific antibodies cannot be detected in the serum. In most subjects, HIV infection is excluded. Possible causes of a false negative ELISA reactivity include:

a.        Primary/acute HIV infection; antibodies are not usually detectable for 2 to 4 weeks after infection. Primary HIV infection is diagnosed by recognition of an acute viral illness (present in 70%), together with detection of HIV-1 p24 antigen (can generally be detected 10 to 14 days after infection). The majority of patients will exhibit antibody reactivity on ELISA and an incomplete western blot (0 to 3 bands);

b.       very advanced HIV infection, when antibody production is substantially impaired (although with assay improvement this is very rare); and

c.        a very unusual strain of HIV-1, usually acquired from a source patient infected outside of Australia.

Molecular tests are not used for the diagnosis of HIV. The HIV DNA detection assay has been withdrawn by the manufacturer. The HIV-1 RNA tests can yield false positive result in healthy adults rarely, and is not recommended (nor licensed).

3.                      Indications

There are numerous clinical settings in which HIV antibody testing should be considered. This includes patients at high risk of HIV-infection by virtue of sexual behaviour or other exposures, subjects with symptoms suggesting acute HIV-infection, subjects with unexplained diseases that might be HIV-related such as thrombocytopenia, dementia, unexplained weight loss, shingles or opportunistic infections.

4.                      Further tests

Testing for HIV that yields indeterminate results should be followed by repeat tests. In general, a biological false positive result will be unchanged or non-reactive 4 to 8 weeks after the initial serum was collected. For a a biological false positive result in a patient with suspected acute HIV infection, repeat serology should be submitted as soon as possible.