HIV
ELISA/Western blot is a set of blood tests used to diagnose chronic infection with human immunodeficiency virus (HIV).
How the Test is
Performed
A blood sample
is needed. For information on how this is done, see: Venipuncture.
How to Prepare
for the Test
No preparation
is necessary.
How the Test
Will Feel
When the needle
is inserted to draw blood, some people feel moderate pain, while others feel
only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the Test is
Performed
Testing for HIV
infection is done for many reasons, including:
- Screening people who want
to be tested
- Screening people in
high-risk groups (men who have sex with men, injection drug users and
their sexual partners, and commercial sex workers)
- Screening people with
certain conditions and infections (such as Kaposi's sarcoma or Pneumocystis jirovecii pneumonia)
- Screening pregnant women
to help prevent them from passing the virus to the baby
- When a patient has an
unusual infection
Normal Results
A negative test
result is normal. However, people with early HIV infection (termed acute HIV
infection or primary HIV infection) often have a negative test result.
What Abnormal
Results Mean
A positive
result on the ELISA screening test does not necessarily
mean that the person has HIV infection. Certain conditions may lead to a false
positive result, such as Lyme disease, syphilis, and lupus.
A positive ELISA
test is always followed by a Western blot test. A positive Western blot
confirms an HIV infection. A negative Western blot test means the ELISA test
was a false positive test. The Western blot test can also be unclear, in which
case more testing is done.
Negative tests
do not rule out HIV infection. There is a period of time (called the
"window period") between HIV infection and the appearance of anti-HIV antibodies that can be measured.
If a person
might have acute or primary HIV infection, and is in the "window
period," a negative HIV ELISA and Western blot will not rule out HIV
infection. More tests for HIV will need to be done.
Risks
Veins and
arteries vary in size from one patient to another and from one side of the body
to the other. Obtaining a blood sample from some people may be more difficult
than from others.
Other risks
associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling
light-headed
- Hematoma (blood
accumulating under the skin)
- Infection (a slight risk
any time the skin is broken)
Considerations
People who are
at high risk (men who have sex with men, injection drug users and their sexual
partners, commercial sex workers) should be regularly tested for HIV.
If the health
care provider suspects early acute HIV infection, other tests (such as HIV
viral load) will be needed to confirm this diagnosis, because the HIV
ELISA/Western blot test will often be negative during this window period.
Alternative
Names
HIV testing
References
Dewar R,
Goldstein D, Maldarelli F. Diagnosis of human immunodeficiency virus infection.
In: Mandell GL, Bennett GE, Dolin R, eds. Principles and Practice of
Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill
Livingstone; 2009:chap 119.
Sax PE, Walker
BD. Immunopathogenesis of human immunodeficiency infection. In: Goldman L,
Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, PA:
Saunders Elsevier; 2007:chap 408.
Posts
