Clinical Laboratory - Microbiology

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Human Immunodeficiency Virus (HIV)

The most common laboratory tests associated with HIV are:

  • EIA for the detection and confirmation of antibodies to HIV proteins

  • HIV-1 RNA quantitation

  • CD4 T-cell counts

Diagnostic Testing for HIV

The most widely used method for the diagnosis of HIV infection is the detection of antibodies to the virus. These serologic methods are based upon the detection of IgG antibody against HIV-1 antigens in the patient's serum (IgM antibodies are not used as they are relatively insensitive).

The standard test for the detection of these antibodies to HIV-1 is the enzyme immunoassay (EIA) screening assay. If a positive result is obtained, then a confirmatory EIA is performed.

It is possible, however, to have a negative EIA in the 'window period' following infection before antibodies can be detected. These patients should have repeat standard serologic testing with EIA performed.

Since infants of HIV positive mothers can have maternal antibodies, then EIA cannot be used. Instead HIV DNA polymerase chain reaction and HIV RNA assays are recommended at ages 14 to 21 days, 1 to 2 months, and 4 to 6 months.

Disease Management

The progression and management of HIV-1 is done by monitoring HIV-1 RNA viral loads and CD4 cell counts.

HIV-1 RNA levels are used as important predictors of the time to progression to acquired immunodeficiency syndrome (AIDS) and in the determination of when to initiate antiretroviral therapy, or change therapy if there is viral resistance. There are a variety of commercial assays available, but most of them are either done through RT-PCR or branched DNA (bDNA) assay. RT-PCR is more sensitive and carries a lower risk of sample contamination. Regardless of which assay is used, it is important to use the same one over time in order to ensure consistent findings.

CD4 T-cell laboratory testing, done through flow cytometry, is essential for the staging and management of HIV. It is used with the HIV-1 RNA load to determine the initiation of antriretroviral therapy (ART), and it is a consistent indicator of treatment response.

A normal adult CD4 count typically falls in a range of 800-1050 cells/mm3, while individuals with HIV have levels that are significantly below this range. When the CD4 count falls below 200 cells/mm3, this indicates the clinical stage of AIDS.

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