A human immunodeficiency virus (HIV) test detects antibodies to HIV in the blood. This determines whether an HIV infection is present (HIV-positive). HIV infects white blood cells called CD4+ cells, which are part of the body's immune system that help fight infections. HIV causes acquired immunodeficiency syndrome (AIDS), a long-term chronic disease that cannot be cured.

After the original infection, it takes between 2 weeks and 6 months for antibodies to HIV to appear in the blood. The period between becoming infected with HIV and the point at which antibodies to HIV can be detected in the blood is called the seroconversion or "window" period. During this period, an HIV-infected person can still spread the disease, even though a test will not detect any antibodies in his or her blood.

Several tests can find antibodies or genetic material (RNA) to the HIV virus. These tests include:

• Enzyme-linked immunosorbent assay (ELISA). This test is usually the first one used to detect infection with HIV. If antibodies to HIV are present (positive), the test is usually repeated to confirm the diagnosis. If ELISA is negative, other tests are not usually needed. This test has a low chance of having a false result after the first few weeks that a person is infected.
• Western blot. This test is more difficult than the ELISA to perform, but it is done to confirm the results of two positive ELISA tests.
• Polymerase chain reaction (PCR). This test finds either the genetic material RNA of HIV. PCR testing is not done as frequently as antibody testing because it requires technical skill and expensive equipment. This test may be done in the days or weeks after exposure to the virus. Genetic material may be found even if other tests are negative for the virus. The PCR test is very useful to find a very recent infection, determine if an HIV infection is present when antibody test results were uncertain, and screen blood or organs for HIV before donation.
• Indirect fluorescent antibody (IFA). This test detects HIV antibodies. Like a Western blot test, it is used to confirm the results of an ELISA. However, it is more expensive than a Western blot test and not commonly used.

Testing is often done at 6 weeks, 3 months, and 6 months after exposure to find out if a person is infected with HIV.