The human immunodeficiency virus (HIV) causes AIDS, a chronic disease that cannot be cured and is usually fatal if left untreated. HIV infects CD4+ cells, which are part of the body's immune system. Two types of HIV have been identified:

• HIV-1. This virus causes almost all of the cases of AIDS worldwide.
• HIV-2. This virus found mostly in west Africa.

People who practice certain behaviors and those who have certain jobs are at higher risk than the general population for becoming infected with human immunodeficiency virus (HIV). Those considered to be members of high-risk groups include:

• People who have had multiple sex partners or who have had a recent episode of a sexually transmitted disease. About 75% of HIV infections are spread through sexual contact.
• People who inject illegal drugs or steroids, especially if they share needles and other drug works.
• People who handle blood or body fluids as a routine part of their job. This includes health professionals, such as doctors, dentists, nurses, blood and lab technicians. It also includes morticians and embalmers. Injecting HIV-infected blood (such as from sharing needles for injecting illegal drugs or from an accidental stick with an HIV-infected needle) causes about 10% of infections.
• People who have hemophilia and others who received transfused blood or other blood products before 1985. Because all donated blood is now screened for HIV antibodies and all blood products are heat treated to destroy HIV, the risk of getting HIV from transfused blood or blood products is now low.

Others who are at increased risk for HIV infection include:

• Infants whose mothers are infected with HIV. The virus can be spread from mother to baby during pregnancy, delivery, or during breast-feeding.
• People who have their ears or other body parts pierced with needles that may not have been properly cleaned (sterilized). However, while this is a rare possibility, it has never been proven that HIV has been spread by piercing.
• People who have tattoos inked with needles that may not have been properly cleaned. However, while this is a rare possibility, it has never been proven that HIV has been spread by tattooing.

The period between becoming infected with HIV and the time when antibodies to HIV can be detected in the blood (seroconversion period) can range from 2 weeks to 6 months. During this period, an HIV-infected person can still spread the disease even though a test does not detect any antibodies in his or her blood.

Several tests can detect antibodies to the HIV virus. These tests are:

• Enzyme-linked immunosorbent assay (ELISA). This screening test is usually the first test used to detect infection with HIV. If antibodies to HIV are present (positive result), the test is usually repeated. Other antibodies can cause an ELISA test result to appear as if HIV antibodies are present when they are not (false-positive result).
• Western blot. This test requires high technical skills. It is more difficult than the ELISA to perform and interpret accurately, but it is less likely to give a false-positive result because it can distinguish HIV antibodies from other antibodies that may react to the ELISA. A Western blot is usually done to confirm the results of two positive ELISA tests. However, the Western blot test can produce inconclusive results in some samples.
• Indirect fluorescent antibody (IFA). This test also detects antibodies made to fight an HIV infection. Like a Western blot test, it is used to confirm the results of an ELISA. However, it is more expensive and more difficult to do than a Western blot test.

A polymerase chain reaction (PCR) test detects the genetic material (ribonucleic acid, or RNA) of HIV, rather than detecting antibodies to HIV. Therefore, PCR can reveal an HIV infection before antibodies can be detected. PCR can also accurately determine whether a baby born to an infected mother has HIV. ELISA and Western blot testing are not always accurate in a newborn baby because the mother's antibodies may be present in the baby's blood for several months after birth.

A test similar to PCR testing, called the branched DNA test, can also be done to detect viral RNA.

These tests are done on a blood sample taken from a vein.

Human Immunodeficiency Virus (HIV) Testing
Results

ELISA results are usually available in 2 to 4 days. Results of the other tests, such as the Western blot or IFA, take longer (1 to 2 weeks).