Laboratory Diagnosis Of HIV

Laboratory Diagnosis Of HIV

Question. Write a short note on the laboratory test for HIV infection.
Answer.

Enzyme-Linked Immunosorbent Assay (ELISA)

ELISA is an initial screening test.

• Mechanism: The HIVus particle, which is an antigen, binds with antibodies to the HIV in an infected human serum. Serum obtained from a blood sample of the patient consists of antibodies that are incorporated into the ELISA plate, which is then washed clear of inactive antibodies that do not bind to antigens. The second layer of antibodies, known as conjugate, should be added to detect the primary antibody in the human serum. Excessive antibodies are then washed and cleared from the plate, and finally, substrate, i.e., chromogen, is added to make reactions occur.
• Positive result: A Positive test means that HIV antigens act on the substrate and change its color.
• Negative result: It means that antibodies will not bind to the HIV antigen, so there is no enzyme to change the color of the substrate.
• ELISA is not 100% reliable, as this is not specific for only HIV antibodies and may also react with other viral antibodies. Ifthe  result of the ELISA test is positive, a more specific western blot method should be carried out.

Laboratory Diagnosis of HIV: Tests, Methods, and Protocols

HIV diagnosis methods

Western Blot Method

This test is given as a follow-up test to a positive ELISA test.

This test is used to eliminate false-positive results of ELISA.

• Mechanism: Here, the viral proteins from the blood sample are passed through the gel. Different proteins migrate via the gel at different speeds, i.e., small proteins migrate at a faster rate while large proteins migrate at a slower rate. As proteins get separated, they are now passed via electric current so that they can be transferred on a solid film strip in order of their speed. Add human serum, and any of the existing HIV antibodies bind to the HIV antigen. Chemical that reacts upon contact with the protein-antibody-enzyme layer change the color of the band. According to the common “3-band rule,” if there is an appearance of three or more bands, HIV antibodies are detected.
• Positive result: HIV-specific nitrocellulose strips are exposed to infected human serum, and goat anti-human antibody strip characteristic band is detected for each of the three groups of viral protein.
• Negative result: No reaction with any viral protein.
• Indeterminate test: Those that react with one of the three groups of antigen.

Viral Load Testing

It measures the presence of the amount of HIV in the human body. This is not the diagnostic test.

This test provides information about the progression of HIV based on CD4 cell count, which predicts the course of the disease and guides the treatment.

• Mechanism: Viral load tests are of two types, i.e., polymerase chain reaction (PCR) and branched DNA (bDNA). It is important to use the same type of viral load test every time and not switch between these two; values between the two are not comparable.
• High viral load: It is between 5000 and one million copies or more.
• Low viral load: It is between 200 and 500 copies
• An increase in the count indicates worsening of the infection, while a decrease in the count is indicative of suppression of the HIV infection.

HIV Diagnosis: Laboratory Testing and Screening Methods

Alternative Diagnostic Test

• Indirect immunofluorescence and microfiltration enzyme assay system: This is a rapid and expensive method.
• p24 antigen capture assay: It detects HIV DNA, which is integrated into host DNA. The test detects HIV antibody within 45 days after exposure.
• Surrogate marker: Absolute CD4 + T cell lymphocyte count correlates best with progression of HIV – I related immune dysfunction. Other serum markers used include neopterin, beta-2-microglobulin, HIV p24 antigen, interleukin-2 receptor, IgA, and impaired delayed-type hypersensitivity.
• OraSure: It absorbs blood-derived fluid from the gingival tissue. It is the only FDA-approved HIV antibody.