Test Overview
A
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
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
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
donation.
- 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. Testing is often done
for people with
risk factors for HIV infection and people who have
symptoms of an HIV infection.
- Screen
blood, blood products, and organ donors to prevent the spread of
HIV.
- Screen pregnant women for HIV infection. The
United States Preventive Services Task Force
recommends all pregnant women be screened. 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.
The Centers for Disease Control and Prevention (CDC)
recommend HIV screening as part of routine blood testing. You and your doctor
can decide if testing is right for you.
This test is not done to
determine if a person has AIDS. A diagnosis of AIDS means a person is
HIV-positive and other problems are present.
How To Prepare
You do not need to do anything before you have this test.
A test for HIV infection can't be done without
your consent. Most doctors offer counseling before and after the test to
discuss:
- How the test is done, what the results mean,
and any other tests that may be done.
- How the diagnosis of an HIV
infection may affect your social, emotional, professional, and financial
outlooks.
- The benefits of early diagnosis and treatment.
Before the test, it is important to tell your doctor how
and where to contact you when your test results are ready. If your doctor has
not contacted you within 1 to 2 weeks of your test, call and ask for your
results.
How It Is Done
The health professional drawing blood
will:
- Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.
- Clean the needle site with
alcohol.
- Put the needle into the vein. More than one needle stick
may be needed.
- Attach a tube to the needle to fill it with
blood.
- Remove the band from your arm when enough blood is
collected.
- Apply a gauze pad or cotton ball over the needle site as
the needle is removed.
- Apply pressure to the site and then a
bandage.
How It Feels
You may feel nothing at all from the
needle puncture, or you may feel a brief sting or pinch as the needle goes
through the skin. Some people feel a stinging pain while the needle is in the
vein. But many people do not feel any pain (or have only minor discomfort)
once the needle is positioned in the vein. The amount of pain you feel depends
on the skill of the health professional drawing the blood, the condition of
your veins, and your sensitivity to pain.
Risks
There is very little risk of complications from
having blood drawn from a vein.
- You may develop a small bruise at the puncture
site. You can reduce the risk of bruising by keeping pressure on the site for
several minutes after the needle is withdrawn.
- In rare cases, the
vein may become inflamed after the blood sample is taken. This condition is
called phlebitis and is usually treated with a warm compress applied several
times daily.
- Continued bleeding can be a problem for people with
bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning
medicines can also make bleeding more likely. If you have bleeding or clotting
problems, or if you take blood-thinning medicine, tell your health professional
before your blood is drawn.
Results
A
human immunodeficiency virus (HIV) test detects
antibodies to HIV or the genetic material (DNA or
RNA) of HIV in blood or another type of sample. This
determines whether an HIV infection is present (HIV-positive). ELISA results
are usually available in 2 to 4 days. Results of the other tests, such as the
Western blot or IFA, take 1 to 2 weeks.
HIV tests| Normal: | No HIV antibodies are found. Normal results are called
negative. |
|---|
| If an antibody test is done during the
seroconversion period and is negative, repeat testing
is needed. Most people have antibodies to HIV within 6 months after becoming
infected. If a repeat test at 6 months is negative, there is no
infection. |
|---|
| A
PCR test to look for genetic material does not detect
any RNA or DNA of HIV. |
|---|
| Uncertain: | Test results do not clearly show whether a person has an
HIV infection. This is usually called an indeterminate result. It may occur
before HIV antibodies develop or when some other type of antibody is
interfering with the results. If this occurs, a PCR test, which detects HIV RNA
or DNA, may be done to see if the virus is present. |
|---|
| A person who still has indeterminate results for 6 months
or longer is called "stable indeterminate" and is not considered to be infected
with HIV. |
|---|
| Abnormal: | HIV antibodies are found. These results are called
positive. |
|---|
| A positive ELISA is repeated using the same blood sample.
If two or more ELISA results are positive, they must be confirmed by a Western
blot or IFA test. |
|---|
| A PCR test finds genetic material (RNA or DNA) of
HIV. |
|---|
What Affects the Test
Reasons you may not be able to
have the test or why the results may not be helpful include:
- Using
corticosteroids.
- Having an antibody test done before a detectable amount of antibodies is present (during the seroconversion period).
- Having an
autoimmune disease, leukemia, or
syphilis.
- Drinking too much alcohol.
What To Think About
- After initial testing, it is important for your
doctor to contact you with the results of your test. Be sure to tell your
doctor how and where to contact you. If your doctor has not contacted you
within 1 to 2 weeks of your test, call and ask for your
results.
- The ELISA is a good screening test, because it is usually
positive when an HIV infection is present. But the ELISA test results can
indicate HIV is present when it is not (false-positive). So the ELISA alone cannot be used to
make a definite diagnosis of HIV infection. No one is considered HIV-positive
until he or she has a positive Western blot, IFA, or PCR test.
- Detecting HIV in a newborn infant is difficult. Until about 18
months of age, even a baby who is not infected may still have HIV antibodies
received from his or her HIV-positive mother. A PCR test may be done to see if
HIV genetic material (RNA or DNA) is present in the baby.
- To be
certain that an HIV infection is not present, a person must test negative for
the virus at least 6 months after the last possible exposure to HIV. Testing is
often done at 6 weeks, 3 months, and 6 months to find out whether a person is
infected.
- Two types of home test kits for HIV have been approved by the U.S. Food and Drug Administration (FDA). If the results from a home test kit show that you have an HIV infection, talk to a doctor. And keep in mind that these test kits sometimes may show that you have HIV when you don't (false-positive result) or may show that you don't have HIV when you do (false-negative result).
- One test kit (called OraQuick) gives you the results right away. For this test, you rub your gums with a swab supplied by the kit. Then you place the swab into a vial of liquid. The test strip on the swab indicates if you have HIV or not.
- Another type of test kit for HIV is a home blood test kit. This
kit provides instructions and materials for collecting a small blood sample by sticking your finger with a lancet. The blood is placed onto a special card that is
then sent to a lab for analysis. You can find out your results over the phone by using an
anonymous code number. Counseling is also available over the phone for people
who use the test kit.
- If you have a
positive test result, contact your sex partners to inform them. They may want
to be tested. You may be able to get help from your local health department to
do this.
- A screening test for HIV infection may also be done on urine
or saliva. An oral HIV test finds antibodies to HIV. Urine testing is rarely
done.
- Oral test kits that find HIV-1 and HIV-2 in saliva have been
approved by the United States Food and Drug Administration (FDA). The test
results are provided the same day. Test results that show an HIV infection
need to be confirmed by a Western blot test.
- Most states require
health professionals, clinics, and hospitals to report confirmed cases of HIV
infection to the state health department. Some states allow anonymous reporting
(the person's name or other identifying information is not provided). Other
states require confidential reporting (identifying information is provided but
only to authorized public health officials). All states must report the numbers
of cases of AIDS, without names or other identifying information, to the
Centers for Disease Control and Prevention (CDC).
- After an HIV infection is present, other tests are done to
determine when to treat the infection and how treatment is working. These tests
include a CD4+ count and the viral load.
- Two types of HIV have been identified.
- HIV-1 causes almost all of the cases of
AIDS worldwide.
- HIV-2 is found mostly in West Africa.
References
Other Works Consulted
- Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
- U.S. Department of Health and Human Services Panel on Antiretroviral Guidelines for Adults and Adolescents (2011). Guidelines for the Use of Antiretroviral Agents in HIV-1-Infected Adults and Adolescents. Available online: http://www.aidsinfo.nih.gov/ContentFiles/AdultandAdolescentGL.pdf.
- U.S. Preventive Services Task Force (2005, amended 2007).
Screening for HIV: U.S. Preventive Services Task Force recommendation statement. Available online:
http://www.uspreventiveservicestaskforce.org/uspstf/uspshivi.htm.
Credits
| By | Healthwise Staff |
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| Primary Medical Reviewer | E. Gregory Thompson, MD - Internal Medicine |
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| Specialist Medical Reviewer | Peter Shalit, MD, PhD - Internal Medicine |
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| Last Revised | November 7, 2012 |
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