AIDS Tests Health Article

Prognostic tests

In addition to tests for viral antigens and antibodies, other blood tests are needed to evaluate and manage patients with HIV disease. The most important of these is the CD4 positive lymphocyte count. This test measures the number of T helper cells in the blood. A CD4 count of less than 200/microL or 14% of the total lymphocyte count in a person who is HIV positive constitutes a diagnosis of AIDS. A falling CD4 positive lymphocyte count parallels a rise in viral replication and correlates with both a risk of opportunistic infection and drug resistance in patients receiving highly active antiviral therapy (HAART).

It is important for doctors treating AIDS patients to measure the CD4 positive lymphocyte count on a regular basis. Experts consulted by the U.S. Public Health Service recommend the following frequency of serum testing based on the patient's CD4+ level:

• CD4+ count more than 600 cells/microL: Every six months.
• CD4+ count between 200-600 cells/microL: Every three months.
• CD4+ count less than 200 cells/microL: Every three months.

When the CD4+ count falls below 200 cells/microL, the doctor will put the patient on a medication regimen to protect him or her against opportunistic infections.

BETA2-MICROGLOBULIN (b2M). Beta₂^-microglobulin is 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 [.beta]₂[.Mu] are found in patients with cancer and other serious diseases, a rising [.beta]₂[.Mu] blood level can be used to measure the progression of AIDS.

GENOTYPIC DRUG RESISTANCE TEST. Genotypic testing can help determine whether specific gene mutations, common in people with HIV, are causing drug resistance and drug failure. The test looks for specific genetic mutations within the virus that are known to cause resistance to anti-viral drugs. For example the drug 3TC, also known as lamivudine (Epivir), is not effective against strains of HIV that have a mutation at a particular position, known as M184V, in their reverse transcriptase enzyme. If the genotypic resistance test shows a mutation at position M184V, it is likely that person is resistant to 3TC and not likely to respond to treatment. Genotypic tests are only useful when the patient is already taking antiviral medication, and the viral load is greater than 1,000 copies per mL of plasma. The cost of the viral drug resistance testing is usually between $300 and $500, and is usually not covered by insurance plans, including Medicare.

PHENOTYPIC DRUG RESISTANCE TESTING. Phenotypic testing directly measures the in vitro sensitivity of a patient's HIV strains to particular drugs and drug combinations. The test measures the concentration of a drug required to inhibit viral replication by 50% and 90%. This is the same method used by researchers to determine whether a drug might be effective against HIV before using it in human clinical trials. Phenotypic testing is a more direct measurement of resistance than genotypic testing. Unlike genotypic testing, phenotypic testing does not require a high viral load, but it is recommended that persons already be taking antiretroviral drugs. The cost is between $700 and $900 and is usually not covered by insurance plans, including Medicare.

AIDS serology in children

Children born to HIV-infected mothers may acquire the infection through the mother's placenta or during the birth process. Public health experts recommend the testing and monitoring of all children born to mothers with HIV. Diagnostic testing in children older than 18 months is similar to adult testing, with ELISA screening confirmed by Western blot. Younger infants can be diagnosed by direct culture of the HIV virus, PCR testing, and p24 antigen testing. These techniques allow a pediatrician to identify 50% of infected children at or near birth, and 95% of cases in infants three to six months of age.

Preparation

In addition to diagnostic testing for HIV infection, many laboratory tests are important to the proper management and treatment of patients with HIV disease and AIDS. These include the complete blood count (CBC), cultures and serological tests for opportunistic infections, Pap smear for cervical cancer, imaging studies, nerve conduction studies, and tests for nutritional status.

Preparation and aftercare are important parts of AIDS diagnostic testing. Doctors are now advised to take the patient's emotional, social, economic, and other circumstances into account and to provide counseling before and after testing. Patients are generally better able to cope with the results if the doctor has spent some time with them before the blood test explaining the basic facts about HIV infection and testing. Many doctors now offer this type of informational counseling before performing the tests.