AIDS tests

AIDS tests, short for acquired immunodeficiency syndrome tests, cover a number of different procedures used to diagnose and treat HIV patients. Some AIDStests are used to diagnose patients or confirm a diagnosis; others are used to measure the progression of the disease or the effectiveness of specific treatment regimens. Some AIDS tests can also be used to screen blood donations for safe use in transfusions.

Diagnostic blood tests for AIDS are usually given to persons in high-risk populations who have been exposed to HIV or who have the early symptoms of AIDS.Most persons infected with HIV will develop a detectable level of antibody within three months of infection. Public health experts recommend testing allchildren born to mothers with HIV.

It is possible to diagnose HIV infection by isolating the virus itself from ablood sample or by demonstrating the presence of HIV antigen in the blood. (An antigen is a protein that causes the immune system to produce antibodies against it.) Isolating the virus itself is expensive, not widely available, and slow. More common are blood tests that work by detecting the presence of antibodies to the HIV virus.

The enzyme-linked immunosorbent assay (ELISA) test involves HIV antigens thatare attached to a plastic surface. A sample of the patient's blood serum isadded. If there are any HIV antibodies in the blood, they will attach to theHIV antigens. After rinsing off excess blood factors, a second antibody is added. This antibody binds with any HIV antibodies that are attached to the HIVantigens. The second antibody also contains a chemical that changes color, indicating the presence of HIV antibodies in the patient's blood.

Occasionally, the ELISA test will be positive for a patient without symptomsof AIDS from a low-risk group. This result may be a false positive. The Western blot or immunoblot test is used to confirm the diagnosis of AIDS. In Western blot testing, HIV antigens are suspended in a flat slab of gel. An electric current run through the gel causes the proteins to separate from one another and move varying distances depending on their molecular size. Afterwards, the gel is pressed against a nylon or nitrocellulose filter, transferring theproteins to the filter. The patient's blood is reacted against the filter, followed by treatment with developing chemicals. If HIV antibodies are present,they show up as a colored patch or blot on the filter.

Immunofluorescent assay (IFA) is sometimes used to confirm ELISA results instead of Western blotting. An IFA test detects the presence of HIV antibody ina sample of the patient's serum by mixing HIV antigen with a fluorescent chemical, adding the blood sample, and observing the reaction under a microscopewith ultraviolet light.

Polymerase chain reaction (PCR) is used to evaluate the very small number ofAIDS patients with false-negative ELISA and Western blot tests. The PCR testcan measure the presence of viral nucleic acids in the patient's blood even when there is no detectable antibody to HIV. However, the overwhelming majority of infected persons will be detected by ELISA screening within one to threemonths of infection.

Blood tests to evaluate patients already diagnosed with HIV infection are also important. Doctors can measure the number or proportion of certain types ofcells in an AIDS patient's blood to see whether and how rapidly the diseaseis progressing, or whether certain treatments are helping the patient. Thesecell count tests include:

  • Complete blood count (CBC).
  • AbsoluteCD4+ lymphocytes. A lymphocyte is a type of white blood cell that is important in an immune response.
  • CD4+ lymphocyte percentage. A white blood cell count that is broken down into categories in this way is called a WBC differential.

The most recent type of blood test for monitoring AIDS patients is the viralload test. It can tell the doctor the speed at which HIV is replicating in the body. The viral load test is based on PCR techniques.

Another test measures beta2-microglobulin (2M), a protein found on the surface of all human cells with a nucleus. It is released into the blood when a cell dies. Although rising blood levels of 2M are found in patients with cancer and other serious diseases, a rising 2M blood level can be used to measure the progression of AIDS. Finally, found in the viral core of HIV, p24 is a protein that can be measured by the ELISA technique. Doctors can use p24 assays to measure the antiviral activityof the patient's medications. However, p24 is consistently present in only 25% of persons infected with HIV.

If the test results indicate that the patient is HIV- positive, he or she will need counseling, information, referral for treatment, and support. Doctorscan either counsel the patient themselves or invite an experienced HIV counselor to discuss the results with the patient. They will also assess the patient's emotional and psychological status.

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