HIV ELISA/western blot

Alternative names
HIV test

This is 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 more specific and will confirm if someone is truly HIV positive (there are other conditions that may inaccurately produce a positive ELISA test result, including lupus, lyme disease, and syphilis).

How the test is performed

Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to 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 band 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.

For an 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.

For infants and children:
The preparation you can provide for this test depends on your child’s age and experience. For specific information regarding how you can prepare your child, see the following topics:

  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

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 (proper treatment can often prevent transmission of the virus to the fetus) and individuals with certain conditions and infections (such as 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 (such as Lyme disease, syphilis, and 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 an interval (called the “window period”) between HIV infection and the appearance of measurable anti-HIV antibodies. So, if someone is suspected of having acute or primary HIV infection and being in the “window period,” a negative HIV ELISA and Western Blot will not rule out HIV infection. Additional testing with HIV viral load or p24 antigen testing will need to be performed.

What the risks are

The risks associated with having blood drawn 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 (men who have sex with men, injection drug users, commercial sex workers, etc.) should be periodically tested for HIV.

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

Johns Hopkins patient information

Last revised: December 7, 2012
by Sharon M. Smith, M.D.

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