Health Encyclopedia

ELISA/Western blot tests for HIV

Definition

HIV ELISA/Western blot is a set of blood tests used to diagnose chronic infection with human immunodeficiency virus (HIV).

Alternative Names

HIV testing

How the test is performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test

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

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 commercial sex workers)
Screening people with certain conditions and infections (such as Kaposi's sarcoma, Pneumocystis pneumonia)
Screening pregnant women to help prevent them from passing the virus to the baby

Normal Values

A negative test result is normal. However, early HIV infection (termed acute HIV infection or primary HIV infection) often results in a negative test.

What abnormal results mean

A positive result on the ELISA screening test does not necessarily mean that the person has HIV infection. 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 Western blot test. A positive Western blot can usually confirm an HIV infection.

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.

What the risks are

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)

Special considerations

People who are at high risk (men who have sex with men, injection drug users, commercial sex workers) should be regularly tested for HIV.

If the health care provider suspects early (acute or primary) HIV infection, other tests (such as HIV viral load) will be needed to confirm this diagnosis, because the HIV ELISA/Western blot will often be negative during this window period.

Review Date:1/22/2008
Reviewed By:Kenneth M. Wener, MD, Department of Infectious Diseases, Lahey Clinic, Burlington, MA. Review provided by VeriMed Healthcare Network.

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