HIV blood screening

HIV blood screening

The risk of transmitting HIV infection to blood transfusion recipients has been drastically reduced by improved donor selection and sensitive serologic screening assays in many countries. In 2000, WHO estimated that 1 million new HIV infections around the world resulted from inadequate blood screening.

Screening tests require a high degree of confidence that HIV is not present, so a combination of antibody (serology), antigen and nucleic acid based tests are used by blood banks in Western countries. The average window period with antibody tests is 22 days. Antigen testing cuts the window period to approximately 16 days and NAT further reduces this period to 12 days. [http://www.fda.gov/bbs/topics/ANSWERS/2001/ANS01103.html FDA 2001]

Public demand in the United States for HIV blood screening arose during the campaign to re elect President Ronald Reagan. In April 1984, U.S. Health and Human Services Secretary Margaret Heckler announced to the world at a press conference that an American scientist, Dr. Robert Gallo, had discovered the "probable cause" of acquired immune deficiency syndrome AIDS: the retrovirus subsequently named Human Immunodeficiency Virus HIV.

The first screening test, an Enzyme Linked Immuno-Sorbency Assay ELISA antibody test, had a high sensitivity but a low specificity. The low specificity of the test is due to cross-reacting antibodies, which attach to HIV particles "by accident", even though the body has never encountered HIV. Antibody tests cannot detect recent HIV infections, because there is a "window period" of several weeks between infection and the production of antibodies. The tests are usually referred to as "reactive" instead of "positive" due to the limitations of the method. A confirmatory test is typically used to determine the donor's status.

The p24 antigen test detects the virus itself, specifically a surface protein (p24) and does not require an immune response. The same methodology (ELISA) is used as the antibody test.

Nucleic Acid Testing (NAT) is performed by several methods, such as Polymerase Chain Reaction (PCR) and Transcription Mediated Amplification (TMA). NAT always detects the genetic material of the virus and is much more specific and sensitive than the ELISA tests since the methods include an amplification of very precisely defined regions of the viral genome. The tests are more expensive, and are typically performed on pooled samples for efficiency, with a second set of tests to identify which sample in the pool was positive.

Similar kinds of testing are used for a variety of other viral diseases that can be transmitted in blood.

Timeline

* March, 1983 FDA issues donor screening guidelines. AIDS high-risk groups should NOT donate.
* April 23, 1984 Secretary of Health and Human Services Margaret Heckler announces discovery of virus believed to cause AIDS; says test to identify contaminated blood will be available in six months.
* March 2, 1985 FDA approves first HIV antibody screening tests.
* March 14, 1996 Food and Drug Administration (FDA) announced approval of the first antigen test kit to screen blood donors for HIV-1, the virus that is responsible for the vast majority of AIDS cases in the United States.
* September 21, 2001 FDA licensed the first nucleic acid test (NAT) systems intended for screening of plasma donors.


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