"How Accurate is the HIV Antibody Test




Q. How accurate are the HIV antibody ELISA and the HIV antibody Western blot?

A. When used together, the results from this two-part testing are greater than 99% accurate. The HIV antibody ELISA is a screening test and the HIV antibody Western blot is a confirmatory test. Results from an HIV antibody ELISA test should never be used alone to report a positive final result.



Q. Do the HIV antibody ELISA and HIV antibody Western blot test for HIV-1 and HIV-2?

A. There are two types of HIV (HIV-1 and HIV-2). Both HIV-1 and HIV-2 have been identified in the United States. The number of known HIV-2 infected persons in the U.S. is less than 100. The estimated number of people in the U.S. infected with HIV-1 is between 650,000 and 900,000.

Some HIV antibody ELISA and HIV antibody Western blot assays detect antibody to both HIV-1 and HIV-2. These are referred to as HIV-1/HIV-2 "combination" tests. Some HIV antibody ELISA and HIV antibody Western blot assays detect antibody primarily to HIV-1 and secondarily to HIV-2. Others detect antibody primarily to HIV-2 and secondarily to HIV-1.



Q. How accurate are HIV antibody tests in detecting the various subtypes of HIV-1?

A. HIV-1 is divided into two groups of subtypes. These two groups are referred to as Group M (major) and Group O (outlier). HIV-1 subtypes of Group M vary, depending on their genetic structure. (3) These include subtypes A through I. In the United States, the predominate HIV-1 subtype is B.

Most antibody tests for detecting HIV-1 were developed with the B subtype of the virus. As the genetic composition of a particular virus diverges from the B subtype, the likelihood that the test will be accurate decreases. Most tests, however, do appear to be able to detect antibody to most strains.



Q. What can cause a false-positive result in an HIV antibody ELISA test?

A. There are many reasons for a false-positive ELISA result. Some of the more common reasons for a false positive are:

Contamination: In a laboratory, samples may be placed in the wrong testing well; wells containing negative samples may be contaminated from adjacent positive wells; plate washers may malfunction. In addition, treated blood and blood abnormalities have been implicated in false positive reactions.
False positive reactions have been reported in 19% of people with hemophilia, 13% of alcoholic patients with hepatitis, and 4% of hemodialysis patients.
Pregnancy. If this is not her first pregnancy, a woman may react positively when she is, in fact, negative.
History of injection drug use.
Cross-reactivity with other retroviruses.


Q. What is the expected false-positive rate for an HIV antibody ELISA?

A. The false-positive rate is 1 to 5 per 100,000 assays."

Stats from CDC

Hardly likely to make much difference to the homosexual/hiv figures!