ELISA/Western blot tests for H... Health Article

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 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.

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 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.

References

Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, PA: Saunders Elsevier; 2007: sect XXIV.