Accuracy of Adolescent HIV Tests
When used together, the results from this two-part testing are greater than 99 percent accurate. The HIV antibody EIA is a screening test and the HIV antibody Western blot is a confirmatory test. Results from an HIV antibody EIA test should never be used alone to report a positive final result.
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.
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. 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.
There are many reasons for a false-positive EIA result. Some of the more common 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 percent of people with hemophilia, 13 percent of alcoholic patients with hepatitis and 4 percent 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.
Yes. When people develop antibodies to HIV, they "seroconvert" from antibody-negative to antibody-positive. Depending upon the circumstances of infection, it is estimated that the development of antibodies to HIV-1 can take between two weeks to six months. During this interval, sometimes referred to as the "window period," a person may test HIV-1 antibody negative and yet be infected with the virus. This is because his/her immune system has not produced enough antibodies for the test to detect.
The term "indeterminate" relative to HIV testing usually refers to the HIV antibody Western blot assay. The HIV antibody Western blot assay is used on two or more specimens found to be reactive by an HIV antibody EIA screening assay. Persons who are not at high risk for HIV infection and do not have symptoms, and yet continue to test indeterminate, usually have a very low probability of being infected with HIV. There are many possible reasons for an indeterminate HIV antibody Western blot assay. Some of these reasons might be:
- Prior blood transfusions, even with non-HIV-1 infected blood
- Prior or current infection with syphilis.
- Prior or current infection with malaria parasites.
- Autoimmune disease (e.g. diabetes, Grave's disease, etc.).
- Infection with other human retroviruses such as HIV-2, HTLV I/II.
- Association with "large animals." Animal trainers and veterinarians are sometimes exposed to viruses which do not cause human disease but may interfere with HIV antibody tests.
- Second or subsequent pregnancies in women.
Whether or not persons who test HIV antibody Western blot indeterminate should be retested depends upon their clinical presentation at the time of testing and what risk factors are present for infection.
"Stable indeterminate" is a term used to describe a situation in which an individual consistently tests indeterminate six months or longer from their last possible exposure. The person should be considered HIV negative unless clinical conditions determine otherwise.
