Getting Tested Overview
HIV tests are used to detect the presence of the antibodies for the human immunodeficiency virus in blood and oral fluids.
Screening donor blood and cellular products
In the USA, since 1985, all blood donations are screened with an ELISA test for HIV-1 and HIV-2, as well as a nucleic acid test. These diagnostic tests are combined with careful donor selection. As of 2001, the risk of transfusion-acquired HIV in the U.S. was approximately one in 2.5 million for each transfusion.
Diagnosis of HIV infection
Tests used for the diagnosis of HIV infection in a particular person require a high degree of both sensitivity and specificity. In the United States, this is achieved by combining two tests for HIV antibodies. If antibodies are detected by an initial test based on the ELISA method, then a second test using the Western blot procedure is used. The combination of these two methods is highly accurate (see below).
The UNAIDS/WHO policy statement on HIV Testing states that conditions under which people undergo HIV testing must be anchored in a human rights approach that pays due respect to ethical principles. According to these principles, the conduct of HIV testing of individuals must be
- Accompanied by counseling (for those who test positive);
- Conducted with the informed consent of the person being tested.
A lot of controversy exists over the ethical obligations of health care providers to inform the sexual partners of individuals infected with HIV that they are at risk of contracting the virus. Some legal jurisdictions permit such disclosure, while others do not. More state funded testing sites are now using confidential forms of testing. This allows for monitoring of infected individuals easily, compared to anonymous testing that has a number attached to the positive test results.
Anonymous testing is testing that has only a number, not the person’s name, attached to the specimen that will be delivered for testing. Items that are confirmed positive will not have the HIV infected individual's name attached to the specimen.
Routine testing recommendation
In the United States, one emerging standard of care is to screen all patients for HIV in all health care settings. In 2006, the Centers for Disease Control announced an initiative for voluntary, routine testing of all Americans aged 13–64 during health care encounters. An estimated 25% of infected individuals were unaware of their status; If successful the effort was expected to reduce new infections by 30% per year. The CDC recommends elimination of requirements for written consent or extensive pre-test counseling, as barriers to widespread routine testing.
Antibody tests may give false negative (no antibodies were detected despite HIV being present) results during the window period, an interval of three weeks to six months between the time of HIV infection and the production of measurable antibodies to HIV seroconversion. Most people develop detectable antibodies approximately 30 days after infection, although some seroconvert later. The vast majority of people (99%) have detectable antibodies by three months after HIV infection; a six-month window is extremely rare with modern antibody testing.
During the window period, an infected person can transmit HIV to others although their HIV infection may not be detectable with an antibody test.
The enzyme-linked immunosorbent assay (ELISA), or enzyme immunoassay (EIA), was the first screening test commonly employed for HIV. It is still commonly used today.
|Western blot test results. The first two strips are negative and positive controls. The rest are actual tests.|
A Western blot test is done to confirm a positive test result on an ELISA test. The results of a Western blot can be positive (the individual is infected with HIV), negative (the individual is not infected with HIV) or indeterminate (cannot tell whether the individual is infected or not). In a generally healthy low-risk population, indeterminate results on Western blot occur on the order of 1 in 5,000 patients.:
Rapid or point-of-care tests
|A woman demonstrates the use of the OraQuick rapid HIV test.|
apid Antibody Tests are intended for use as a point-of-care test to aid in the diagnosis of HIV infection.
If no antibodies to HIV are detected, this does not mean the person has not been infected with HIV. It may take several months after HIV infection for the antibody response to reach detectable levels, during which time rapid testing for antibodies to HIV will not be indicative of true infection status. For most people, HIV antibodies reach a detectable level after two to six weeks.
Although these tests are very accurate, false positives do occur. Any positive test result.
Interpreting antibody tests
ELISA testing alone cannot be used to diagnose HIV, even if the test suggests a high probability that antibody to HIV-1 is present. In the United States, such ELISA results are not reported as "positive" unless confirmed by a Western Blot.
Rare false positive results due to factors unrelated to HIV exposure are found more often with the ELISA test than with the Western Blot. False positives may be associated with medical conditions such as recent acute illnesses and allergies. A false positive result does not indicate a condition of significant risk to health. When the ELISA test is combined with Western Blot, the rate of false positives is extremely low, and diagnostic accuracy is very high (see below).
Risk history, and clinical judgement should be included in the assessment, and a confirmation test (Western blot) should be administered. An individual with an inconclusive test should be re-tested at a later date.
Accuracy of HIV testing
Modern HIV testing is highly accurate. The evidence regarding the risks and benefits of HIV screening was reviewed in July 2005 by the U.S. Preventive Services Task Force. The authors concluded that:
...the use of repeatedly reactive enzyme immunoassay followed by confirmatory Western blot or immunofluorescent assay remains the standard method for diagnosing HIV-1 infection. A large study of HIV testing in 752 U.S. laboratories reported a sensitivity of 99.7% and specificity of 98.5% for enzyme immunoassay, and studies in U.S. blood donors reported specificities of 99.8% and greater than 99.99%. With confirmatory Western blot, the chance of a false-positive identification in a low-prevalence setting is about 1 in 250 000 (95% CI, 1 in 173 000 to 1 in 379 000).
Other studies have confirmed the accuracy of current methods of HIV testing in the United States, reporting false-positive rates of 0.0004% to 0.0007% and false-negative rates of 0.003% in the general population.
For more information on HIV testing please click the “Testing Providers” tab or search by zip code under “Locate a Testing Center.”
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