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Blood test
Blood test

HIV ELISA/western blot
Definition:
A set of blood tests used in the diagnosis of chronic infection with human immunodeficiency virus (HIV). The HIV ELISA is a screening test in the diagnosis of HIV infection. If this test is positive, it must be confirmed with a second test called the Western Blot which is a more specific test and will confirm if someone is truly HIV positive, as there are other conditions which may give a false positive ELISA screening test (eg. lupus, lyme disease, syphilis).

Alternative Names:
HIV test

How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test:

No physical preparation is necessary. HIV testing requires written consent in most U.S. states.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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 is performed and recommended for many reasons including screening in high-risk groups (men who have sex with men, injection drug users, commercial sex workers, etc); pregnant women (given the ability to decrease rates of transmission to the fetus with appropriate therapy) and individuals with certain conditions and infections (eg. Kaposi's sarcoma, Pneumocystis carinii pneumonia).
Normal Values:
A negative test result is normal - however, early HIV infection (termed acute HIV infection or primary HIV infection) is often associated with a negative test.
What abnormal results mean:

The ELISA is used as a screening test. A positive result does not necessarily mean that the subject has HIV infection as there are certain conditions that may lead to a false positive result (eg. lyme disease, syphilis, lupus). A positive ELISA test is always followed by a confirmatory test termed Western Blot. A positive western blot is generally regarded as conclusive for an HIV infection. Negative tests do not necessarily rule out HIV infection, because there is a time interval between HIV infection and the appearance of measurable anti-HIV antibodies (the so-called "window period"). Hence, if an individual is suspected of having acute or primary HIV infection and therefore of being in the "window period", a negative HIV ELISA and Western Blot will not rule out this diagnosis. Additional testing with HIV viral load or p24 antigen testing will need to be performed.

What the risks are:
  • excessive bleeding
  • fainting or feeling lightheaded
  • hematoma (blood accumulating under the skin)
  • infection (a slight risk any time the skin is broken)
  • multiple punctures to locate veins
Special considerations:

Individuals at high risk for HIV disease (men who have sex with men, injection drug users, commercial sex workers etc) should be periodically tested for HIV.

If early (acute HIV or primary HIV infection) is suspected - additional studies will be needed to confirm this diagnosis (eg. HIV viral load, p24 antigen testing), as the HIV ELISA/Western blot will often be negative during the window period.

Review Date: 3/1/2002
Reviewed By: Rocio Hurtado, M.D., Infectious Diseases Division, Massachusetts General Hospital and Brigham and Women's Hospital, Boston, MA. Review provided by VeriMed Healthcare Network.
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