June 2007

HIV Testing 101: An Introduction to Testing Modalities

Since 1985 testing for the Human Immunodeficiency Virus (HIV) has been available. After an individual is exposed to the virus, the body’s immune system recognizes the virus as a foreign substance and begins to make antibodies. Tests determine a patients HIV status by checking for the presence of antibodies.

HIV Antibody Test

The most common and accurate test to determine if a patient has been infected with HIV is the HIV antibody test. The test consists of a screening test called an Enzyme Linked Immuno-Sorbent Assay (ELISA) or Enzyme Immunosorbent Assay (EIA) and a confirmatory test. The two confirmatory tests used are the Western Blot Assay (WB) or an Indirect Immunofluorescense Assay (IFA). Results are given to patients after both screening and confirmatory test have shown a positive result. Positive results are 99% accurate and negative results are 99% accurate. It is recommended that patients wait at least 3 months from the time of possible infection to increase the accuracy of the test results. This time period allows for the "window period" where a patient who is infected has not yet produced detectable levels of HIV antibodies. Patients usually develop detectable antibodies within 4-6 weeks after infection with HIV. Rarely, a patient will need longer than three months to develop detectable antibodies to HIV.

Home self-collection test for HIV

Home Access™ is the only FDA approved home self-collection test on the market. Other test are available illegally but should not be used due to very poor accuracy. Home HIV test are self-collection devices. A patient collects a sample of their blood and sends it to a laboratory to be tested. The sample is evaluated using the screening and confirmatory tests. Patients call a toll-free number and use an identification number to receive the results. Home tests provide the advantages of anonymity and convenience. Disadvantages of the home test include the cost (~$45), no in-person counseling, and the inability of health care professions to help assure notification of partners.

The p24 Antigen Test

p24 is a protein specific for HIV. The p24 antigen test is usually only positive for one to 3-4 weeks after a person is infected with HIV. After the narrow window of detectible levels the test will be negative even in people infected with HIV. The p24 test use is limited in HIV infection, but is utilized as an additional test by blood banks to reduce the possibility of HIV infected blood being used in transfusions.

Viral Load/PCR Testing

Viral load testing is a direct measurement of the amount of HIV in the blood. Polymerase Chain reaction (PCR) tests are used to track how a patient is responding to antiviral treatment and to assist in clinical decisions. PCR testing has not been approved by the FDA as diagnostic tool. Viral load testing has become a standard method of monitoring viral activity. Low numbers correspond to a low viral count. “Undetectable” viral loads do not indicate a patient is no longer infected. Most tests can only detect approximately 40 viral particles per milliliter of blood. Therefore, “undetectable” means an individual has a viral load less then the amount the test can measure.

Additional testing methods

Patients recently infected have lower overall levels of HIV antibodies and may test negative when the serum is diluted and incubated for a shorter than normal period of time. This “less sensitive” ELISA test is currently being used to assess if the infected individual is likely to have recently acquired HIV (within the last 4-6 months) or not. This test is used after the patient has been confirmed to be HIV positive using the regular HIV antibody test and helps providers and patients make medication treatment decisions and focus partner notification.

Testing for HIV using oral fluids and urine has been approved by the FDA. Saliva and other body fluids contain the HIV antibodies, but do NOT contain the virus itself. The oral fluid and urine tests use a screening and confirmatory test. Urine test accuracy is less than that of blood and oral fluid tests.

Rapid testing using a small sample of blood and produces results in approximately 15-30 minutes. This test uses technology known as Single Use Diagnostic Systems (SUDS). It is 99% accurate when used 3 months after possible exposure, however, only negative results can be reported. Tentative reporting of positive results can be given to a patient as positive results still need to be confirmed using the WB or IFA.

References

Update on HIV Testing Accessed 5/14/07

How HIV Test Work CDC Accessed 5/14/07

Rapid HIV Testing CDC Accessed 5/14/07

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