Human Immunodeficiency Virus (HIV) Test
human immunodeficiency virus (HIV) test detects
antibodies to HIV or the genetic material (DNA or
RNA) of HIV in the blood or another type of sample.
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 can progress to
acquired immunodeficiency syndrome (AIDS).
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 to or genetic material (RNA) of the HIV virus. These tests
- 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
- Polymerase chain reaction (PCR). This
test finds either the
RNA of the HIV virus or the HIV
DNA in white blood cells infected with the virus. 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
- Indirect fluorescent antibody (IFA). This test detects HIV antibodies using a special fluorescent dye and a microscope. This test may be used to confirm the results of an ELISA test.
Testing is often done at 6 weeks, 3 months, and 6 months
after exposure to find out if a person is infected with HIV.
Why It Is Done
A test for the human immunodeficiency
virus (HIV) is done to:
- Detect an HIV infection.
blood, blood products, and organ donors to prevent the spread of
- Screen pregnant women for HIV infection. Pregnant women who are infected with
HIV and receive treatment are less likely to pass the infection on to their
babies than are women who do not receive treatment.
- Find out if a
baby born to an HIV-positive woman also is infected with HIV.
A PCR test is often done in this case because the baby may
get antibodies against HIV from the mother and yet not be infected.