Health Encyclopedia - Test
ELISA/Western blot tests for HIV
HIV ELISA/Western blot is a set of blood tests used to diagnose chronic infection with human immunodeficiency virus (HIV).
How the test is performed
A blood sample is needed. For information on how this is done, see: Venipuncture.
How to prepare for the test
No preparation is necessary.
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 their sexual partners, and commercial sex workers)
- Screening people with certain conditions and infections (such as Kaposi's sarcoma or Pneumocystis jirovecii 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, people with early HIV infection (termed acute HIV infection or primary HIV infection) often have a negative test result.
What abnormal results mean
A positive result on the ELISA screening test does not necessarily mean that the person has HIV infection. Certain conditions 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 unclear, in which case more testing is done.
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)
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 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.
Dewar R, Goldstein D, Maldarelli F. Diagnosis of human immunodeficiency virus infection. In: Mandell GL, Bennett GE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 119.
Sax PE, Walker BD. Immunopathogenesis of human immunodeficiency infection. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, PA: Saunders Elsevier; 2007:chap 408.
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.