Topic III - Types of Testing

HIV Testing

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HIV Antibody Tests

Antibodies are produced by the immune system to fight infection. The immune system usually takes 2 to 8 weeks to produce antibodies against HIV. Current antibody tests may identify infection as early as 21 days, but not all testing providers use the most current testing technology. Antibody tests can be used as an initial screening for HIV or as supplemental tests to assist in making a diagnosis.


Antibody screening tests (immunoassays) are the most commonly used HIV tests. Immunoassays (IA) conducted at the point of care are capable of providing preliminary rapid results in 20 minutes or less. Recently developed laboratory-based enzyme immunoassays (EIA) or chemiluminescent immunoassays (CIA) can identify infection as early as 3 weeks after infection. Screening immunoassays have been designed to detect HIV-1 and/or HIV-2 antibodies in blood, oral fluid and urine specimens. Reactive/positive screening test results are considered preliminary.. Supplemental testing is required to confirm the infection.

  • Blood-based screening tests: This type of test detects antibodies in whole blood, plasma and/or serum. Most point of care rapid tests can detect antibodies in whole blood collected via finger stick or venipuncture. If a point of care test is unavailable, plasma or serum specimens may be collected at the test site and submitted to a lab for processing. Blood-based screening tests are used in public health clinics, medical offices and community outreach settings.
  • Oral fluid: This test detects HIV antibodies in fluid in the mouth called oral mucosal transudate. The oral specimen rapid test kit uses a special swab to collect the fluid around the gums of the mouth. If a point of care test is not available, an oral fluid collection device may be sent to a licensed lab for processing. Public health clinics and community outreach agencies may offer HIV testing using oral fluid specimens.
  • Urine: Only one FDA approved enzyme immunoassay (EIA) is available on the market to detect HIV antibodies in urine (Maxim HIV-1 Urine EIA manufactured by Maxim Biomedical, Inc.). The availability of the urine HIV test within physician offices in Washington State is limited.


Supplemental antibody tests: Positive/reactive screening tests have been historically confirmed by laboratory-based antibody tests such as the Western blot (WB) or immunofluorescence assays (IFA). The HIV-1 Western blot and IFA are no longer part of the recommended algorithm for testing serum or plasma specimens. Both tests should be replaced by newer technology capable of confirming early infections. It is recommended to use a combination antigen / antibody immunoassay (see below) capable of identifying infection earlier as the initial screening test for HIV. A supplemental antibody test such as the Multispot HIV-1/HIV-2 (Bio-Rad Laboratories) capable of differentiating HIV-1 antibodies from HIV-2 antibodies is recommended to confirm an HIV diagnosis. Negative or indeterminate results on the differentiation test may require supplemental viral RNA testing (see below).


At home HIV test kits: There are currently two home HIV test kits approved by the United States Food and Drug Administration (FDA). The following two tests have been approved by the FDA to detect antibodies against HIV:

  • Home Access HIV-1 Test System (manufactured by Home Access Health Corporation): requires a few drops of blood, which is mailed to a licensed lab in a safe mailer. If the screening test for HIV-1 antibodies is reactive, supplemental testing is performed by the lab to confirm the result. The client calls the company to obtain results using an anonymous code number. Confidential counseling and referral services are available from the manufacturer. Clinical studies have shown the sensitivity of the test to be over 99.9% (i.e. an estimated 99.9% of infected individuals who test will receive a reactive result when the kit is used according to the manufacturer’s guidelines).
  • The OraQuick In-Home HIV Test (manufactured by OraSure Technologies) is an oral swab in-home test for HIV-1 and HIV-2. The oral test kit uses a special collection device that looks like a toothbrush. It tests for antibodies against HIV-1 and HIV-2. Results are available in 20 minutes; shipping the sample is not required. Positive results are preliminary and supplemental testing is required. The company offers a confidential 24/7 support center. The FDA states that an estimated 1 out of 12 people tested with the OraQuick in-Home HIV test will test negative when they are actually positive (i.e. receive a “false negative” result). A false positive result is also possible, but in rare cases.

 

Combination Antigen (Ag) / Antibody (Ab) Tests:


Recently developed “4th generation” immunoassays are capable of detecting antibodies against the Human Immunodeficiency Virus and parts of the virus itself (antigen). Combination tests are capable of identifying HIV infections earlier than tests that detect only antibodies as described above. Viral antigens such as the viral capsid (core) protein p24 may be detectable 2 weeks after infection. However, the p24 protein may be undetectable after the body begins to produce antibodies to the virus. Therefore, combination tests capable of detecting both virus antigens and antibodies to HIV were developed. Combination antigen and antibody tests require a blood sample (serum, plasma, or whole blood) to be collected.


Nucleic Acid Tests:


Nucleic acid tests detect the genetic material of the virus. Ribonucleic acid (RNA) tests are capable of identifying HIV in the bloodstream around 10 days after infection. FDA approved RNA tests must be performed by licensed laboratories. A blood (plasma) specimen is required to perform nucleic acid testing.

 

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