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.

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)

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.

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

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.