AIDS Tests Health Article

Definition

AIDS tests, short for acquired immunodeficiency syndrome tests, cover a number of different procedures used in the diagnosis and treatment of HIV-infected patients. Tests that measure antibodies to the human immunodeficiency virus (HIV) are called AIDS serology tests. Serology is the branch of immunology that deals with the identification and measurement of antibodies in serum which indicate the presence of disease or immunity to disease. Serum is the normally clear light yellow noncellular portion of blood that forms after the sample is allowed to clot. Some AIDS tests measure HIV antigens or nucleic acid rather than antibodies produced in response to HIV infection. AIDS tests evaluate the presence of HIV in blood serum, and the effects of HIV infection on the patient's immune system.

Purpose

AIDS serology tests have several uses. Some AIDS tests 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.

In order to understand the different purposes of the blood tests used for AIDS patients, it is helpful to understand how HIV infection affects the immune system. HIV is a retrovirus that enters the blood stream of a new host in the following ways:

• sexual contact, including oral and anal intercourse
• entry of HIV infected body fluids (such as blood or urine) through a cut or break in the skin
• transmission during pregnancy
• using or being pricked by a needle that had previously been used by or on an infected person
• transfusion of infected blood products

A retrovirus is a virus that contains two identical strands of RNA and a unique enzyme called reverse transcriptase that converts the viral RNA to DNA within the host cell. Another viral enzyme called integrase inserts this proviral DNA into the host cell genome. Other viral proteins control the process of transcription which forms RNA copies of the inserted DNA, production of structural viral proteins, assembly of immature virus particles, maturation, and release (budding) from the host cell. The entire process takes 12-24 hours.

The primary host cell for HIV is the T helper cell. The HIV envelope contains a glycoprotein called gp120, which binds to a surface molecule on the T helper cell called CD4. Other types of lymphocytes which lack the CD4 molecule are not infected. In addition to T helper cells, HIV can infect phagocytic cells (macrophages, monocytes, and dendritic cells) which serve as reservoirs and spread the virus throughout the body. The virus can also infect certain tissue cells such as neurons, which in part accounts for some of the underlying pathology of HIV disease.

HIV disease begins as a flu-like illness two to six weeks after infection which subsides without treatment. Antibodies to the viral envelope appear eight to 12 weeks after infection. Most patients enter a phase of clinical latency, which on average lasts eight to 10 years. This is followed by gradual loss of CD4 positive lymphocytes (T helper cells). T helper cells produce a substance called interleukin-2 (IL-2), which stimulates other cells (T cells and B cells) in the human immune system to respond to infections. Without the IL-2, the immune response collapses and patients become susceptible to a wide range of infections. Depletion of T helper cells signals the onset of opportunistic infections, malignancy, dementia, and a constellation of other diseases associated with AIDS.

There are no medical restrictions on administering AIDS tests. Most tests are performed on blood, but a screening test using urine is available. Health care professionals should always follow standard precautions recommended by the Centers for Disease Control (CDC) to reduce the possibility of accidental needlestick injury or exposure to the patient's blood and body fluids. This includes wearing latex gloves, washing hands before and following venipuncture, and using disposable needles and safety devices.