|Blood Safety and HIV: UNAIDS Technical Update (UNAIDS, 1997, 8 p.)|
|At a Glance|
Millions of lives are saved each year through blood transfusions. Various shortcomings, though, in the way blood is collected, tested (or not tested) for infections such as HIV, and transfused, mean that people in many countries have an increased risk of becoming infected with HIV and other diseases through transfusions. It is estimated that between 5% and 10% of all HIV infections worldwide have been acquired through transfusion of contaminated blood and blood products. If the proper steps are taken, such infections can be easily prevented.
Apart from HIV infection, other diseases can also be transmitted through transfusions of blood or blood products. These include two other viral infections - hepatitis B (HBV) and hepatitis C (HCV); syphilis; malaria, which is endemic in many tropical areas; HTLV-I/II in endemic areas; and Chagas disease, which is common in rural areas of South and Central America.
While 80% of the world s population live in developing countries, people in developing countries are supported by only 20% of the world s blood supply.
In developing countries, most transfusions are given to: women, to treat haemorrhage as a complication of pregnancy; children with severe anaemia; and serious trauma victims.
Each year, up to 4 million blood donations worldwide are not tested for HIV or HBV. Very few donations are tested for HCV.
While in developed countries, whole blood donations are mainly from voluntary unpaid donors, in developing countries 80% of the donated blood comes from paid or replacement donors, who in general are more likely to carry transfusion-transmissible infections (TTIs). As a result, the risk of disease from blood transfusions is much higher in developing countries.
Whole blood and blood products
Whole blood can be separated into blood products which consist of plasma and cellular components (red cells, white cells and platelets). Plasma contains water, electrolytes, proteins and clotting (coagulation) factors - the latter being valuable for certain medical conditions such as haemophilia. Plasma needs to be frozen within 6-8 hours of the collection of whole blood and maintained, frozen solid, at a temperature of -20°C or colder.
Whole blood and packed red cells must always be stored at a temperature of between +2°C and +8°C. If special anticoagulants are used, blood stored at this temperature can be kept for up to 35 days. Platelets must be kept at 20°C ± 2°C, and stored no longer than 5 days. Plasma derivatives may be shared internationally, while whole blood and red cells, with a short shelf-life, are usually used nationally.
Screening of blood
The process of testing blood for transfusion-transmissible agents is known as screening. Implicit in the practice of screening blood for infectious agents is the concept of good laboratory practice, to ensure correct blood grouping, compatibility of donor and recipient, and all processes leading to the provision of safe and effective blood and blood products.
In the case of HIV, several types of tests - based on different technologies - exist to detect HIV antibodies. Detailed information on the types of HIV tests and testing strategies is available in the UNAIDS Technical Update on HIV Testing Methods.
In general, ELISAs are more suitable for blood banks processing daily a large number of blood units, while simple and/or rapid tests are more appropriate for smaller blood banks with a limited number of donations each day. Several simple/rapid tests perform just as well as ELISAs and are also highly appropriate for use in emergency situations.
The test selected for screening donated blood units should preferably be a combined HIV-1/HIV-2 test which is highly sensitive. A test with a high sensitivity will not produce - or will only rarely produce - false-negative results, which is important for safeguarding the blood supply.
HIV infection is most frequently diagnosed by detecting antibodies which the body produces as it tries to resist the virus. These antibodies usually begin to be produced within 3 to 8 weeks after the time of infection. The period following infection but before the antibodies become detectable is known as the window period. If a person donates blood during this window period, the usual antibody test may give a false-negative result even though the person is infected. Increasingly sensitive anti-HIV tests have shortened the window period to 21 days.
Tests also exist that detect the virus itself rather than antibodies to it; these are called HIV p24 antigen tests. With a test of this kind it is sometimes possible to detect HIV p24 antigen during the window period, if by coincidence the blood donor happens to be tested during the short peak of high levels of circulating virus particles. Although in theory the HIV antigen test can shorten the window period by an additional 6 days, its use is of limited value and there still remains a window of one to two weeks. (See Bush & Alter, 1995 in the Key Materials.)
Several studies have shown that for minimizing the TTI risk, careful selection of donors is more efficient than HIV antigen testing. In addition, a well-functioning quality assurance programme will reduce the possibility of false-negative results resulting from technical errors. In most settings, HIV p24 antigen testing of the blood supply is not cost-effective and is not recommended by WHO.
Donated blood should be tested not only for HIV but also for syphilis, hepatitis B surface antigen, and - if funds permit - hepatitis C antibodies. According to their geographic prevalence, screening for Chagas disease, HTLV-I/II and other TTIs may also need to be carried out.
Some industrialized countries have suggested ending screening for syphilis because its prevalence in their donor populations is very low and because the agent does not survive if the blood is stored at between +4°C and +8°C for at least 72 hours. However, in many countries, blood is stored for only a short time before being transfused. In some of these countries the prevalence of syphilis in blood donors is high. Although syphilis is not a marker for HIV infection, it does indicate donors who have not deferred themselves, yet who are at risk of sexually transmitted diseases, including HIV. Thus the syphilis test serves as a marker of donor suitability.
Units of donated blood yielding reactive or indeterminate test results must be considered as probably infected and must be discarded according to universal safety instructions. If a blood donor is to be notified of a test result, reactive screening results must be confirmed (see WHO/UNAIDS). Revised recommendations for the selection and use of HIV antibody tests. Weekly Epidemiological Record, 1997; 72, 12 (March 21): 81-88.