ELISA/Western blot tests for HIV
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.
No preparation is necessary.
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.
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, Pneumocystis pneumonia )
- Screening pregnant women to help prevent them from passing the virus to the baby
- When a patient has an unusual infection
A negative test result is normal. However, early HIV infection (termed acute HIV infection or primary HIV infection) often results in a negative test.
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 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 “indeterminate,” in which case additional testing is done to clarify the situation.
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.
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)
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 or primary) 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.
Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, PA: Saunders Elsevier; 2007: sect XXIV.
Review Date: 6/5/2009
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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