LABORATORY DIAGNOSIS OF HIV VIRUS

T.U 2059,2060,2062,2063

LABORATORY DIAGNOSIS OF HIV VIRUS

Tests to identify HIV infection can be divided into different categories: virus cultivation, antigen detection and viral genome amplification (PCR).

Virus cultivation

Virus can be isolated from infected persons in most phases of the infection. Peripheral blood mononuclear cells (PBMC) can be co-cultivated with activated PBMC from HIV-negative donors in the presence of IL-2. A positive result is recognized by appearance of virus antigen (p 24) or reverse transcriptase activity in the culture medium.

Antigen/Antibody detection

Antibodies usually become detectable from 3 to 12 weeks after infection. As a rule, an infected person remains antibody-positive for life, but antibody titres often fall in patients with AIDS. The most widely applied tests are the indirect and the competitive ELISA, using mostly a mixture of viral antigens. It is recommended that confirmatory tests are carried out to exclude the possibility of false positive results. These are either variations of ELISA tests or Western blot analysis of antibody specificity.

Viral genome amplification (PCR).

The PCR technique represents a major advance in the diagnosis of HIV infection. This powerful technique can amplify target DNA present in minute amounts. It is therefore useful for early detection of HIV in infants born to infected mothers, since the presence of maternal IgG antibodies excludes serological testing during the first months after birth. Quantitative determination of plasma viraemia (virion RNA) by reverse transcription PCR (RT-PCR) has become a major tool to follow the progression of HIV infection in untreated patients and to monitor the effects of antiviral chemotherapy in patients.

CD4+ lymphocyte count

A hallmark of chronic HIV infection is the depletion of CD4+ lymphocytes and loss of these cells is closely associated with acquisition of the characteristic opportunistic infections. The monitoring of CD4+ lymphocyte count is therefore an important determinant for clinical staging, initiation of antiviral therapy and PCP prophylaxis. However, the present knowledge that HIV is actively replicating in lymphatic organs throughout the infection raises the question of whether the peripheral blood, containing 2% of the total T-cell population, gives a representative picture of the pathogenic process. Plasma viraemia, together with CD4 counts, has therefore come to play a more important role in deciding when to initiate antiviral therapy.