TESTING OF DONOR BLOOD FOR INFECTIOUS DISEASES

Background

An important step in ensuring safety is the screening of donated blood for possible infectious diseases. Today, eight tests for infectious diseases are conducted on each unit of donated blood. Tests for hepatitis B and syphilis were in place before 1985. Since then, tests for Human Immunodeficiency Virus (HIV), Human T-Cell Virus (HTLV) and the Hepatitis C Virus (HCV) have been added. The following seven required tests are performed on each unit of blood:

Hepatitis B Surface Antigen (HBsAg)

The hepatitis B virus, which mainly infects the liver, has an inner core and an outer envelope (the surface). The HBsAg test is done to detect if portions of an individuals outer envelope may be infected with the hepatitis B virus. Hepatitis B can cause inflammation of the liver, and in the earliest stage of the disease, infected people may feel poorly or even have yellow discoloration of the skin, known as jaundice. Fortunately, most patients recover completely and test negative for HBsAg approximately four months after the symptomatic illness. A small percentage of people may become chronic carriers of the virus, and in these cases, the test may remain positive.

Antibodies to the Hepatitis B Core (Anti-HBc)

The anti-HBc test detects an antibody of the hepatitis B virus that is produced during and after infection. If an individual has a positive anti-HBc test, but the HBsAg test is negative, it usually means that the person once had hepatitis B, but has recovered from the infection. Of the individuals with a positive test for anti-HBc, a small percentage have not been exposed to the hepatitis B virus. This kind of test result is called a false positive, and although the individual may be permanently deferred from donating blood, it is unlikely that the person’s health will be negatively affected.

Antibodies to the Hepatitis C Virus (Anti-HCV)

This test is used to screen donors for the hepatitis C virus (HCV). It works by detecting antibodies directed against portions of the virus called antigens. HCV causes inflammation of the liver, and up to 80% of those exposed to the virus develop chronic hepatitis. Eventually, up to 20% of people with HCV may develop cirrhosis of the liver or other severe liver diseases. As in other forms of hepatitis, individuals may be infected with the virus, but may not realize they are carriers since they do not display any symptoms. Drug therapy for persons with chronic hepatitis C may be effective in some cases.

Antibodies to the Human Immunodefiency Virus, Types 1 and 2 (Anti-HIV-1, -2)

This test is designed to detect antibodies directed against antigens of the HIV-1 or HIV-2 viruses. HIV-1 is much more common in the United States, whereas HIV-2 is more prevalent in Western Africa. However, donors are tested for both viruses because both are transmitted by infected blood, and a few cases of HIV-2 have been identified in U.S. residents. Both of these viruses can cause acquired immunodeficiency syndrome, or AIDS.

HIV-1 p24 Antigen

This test screens for antigens of the HIV-1 virus. The extra safety added by doing this test derives from its ability to detect HIV-1 infection a week earlier than the antibody test. Thus, the HIV-1 infection can be identified sooner, and the risk of getting HIV-1 from a blood transfusion is decreased to 1 in 825,000 units of screened blood (As reported in a December 1995 study published in the New England Journal of Medicine).

Antibodies to Human T-Cell Lymphotropic Virus, Types I and II (Anti-HTLV-I, -II)

This test screens for antibodies directed against portions of the HTLV-I and HTLV-II viruses. Both of these viruses are relatively uncommon in the United States, but do occur more frequently in certain populations. HTLV-I is more common in Japan and the Caribbean. The disease can persist for a lifetime, but usually does not cause major illnesses in most people who are infected. In rare instances, the virus may eventually cause nervous system disease or an unusual type of leukemia. HTLV-II infections are usually associated with intravenous drug usage, especially among people who share needles or syringes. This virus may also eventually cause nervous system disease, but only in a small percentage of infected people.

Syphilis

This test is done to detect evidence of infection with the bacterium, called spirochete, that causes syphilis. Blood centers began testing for this shortly after World War II, when syphilis rates in the general population were much higher. The risk of transmitting syphilis through a blood transfusion is exceedingly small because the infection is very rare in blood donors, and also fragile and unlikely to survive blood storage conditions.

All of the above tests are referred to as screening tests, and are designed to detect as many infections as possible. Because these tests are so sensitive, some donors may have a false positive result, even if they were never exposed to the particular infection. In order to sort out true infections from false positive test results, screening tests that are reactive may be followed up with more specific tests called confirmatory tests. Thus, confirmatory tests help determine whether a donor is truly infected.

If the test result from a donated unit of blood is abnormal for any of these disease markers, the unit is discarded.