Testing of Donor Blood for Infectious Disease
The AABB and its members are committed to ensuring a safe blood supply for everyone who may need transfusions. An important step in ensuring safety is the screening of donated blood for infectious diseases. Today, nine 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-1 and HIV-2), human T-lymphotropic virus (HTLV-I and -II) and the hepatitis C virus (HCV) have been added. The following 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 detects the outer envelope, identifying an individual 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 ill or even have yellow discoloration of the skin or eyes, a condition known as jaundice. Fortunately, most patients recover completely and test negative for HBsAg within a few months after the illness. A small percentage of people become chronic carriers of the virus, and in these cases, the test may remain positive for years. Chronically infected people can develop severe liver disease as time passes, and need to be followed carefully by an experienced doctor.
Antibodies to the Hepatitis B Core (Anti-HBc)
The anti-HBc test detects an antibody to 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 may mean that the person once had hepatitis B, but has recovered from the infection. Of the individuals with a positive test for anti-HBc, many 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. (Note: This antibody is not produced following vaccination against hepatitis B. Hepatitis B vaccination, by itself, will rarely cause the HbsAg test to be positive for a few days after the shots.)
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 manufactured by the body in reaction to portions of the virus called antigens. HCV causes inflammation of the liver, and up to 80 percent of those exposed to the virus develop chronic infection. Eventually, up to 20 percent 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 have any symptoms. Because of the risk of serious illness, people with HCV need to be followed closely by a physician with experience evaluating this infection.
Antibodies to the Human Immunodeficiency 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, while HIV-2 is prevalent in Western Africa. Donors are tested for both viruses because both are transmitted by infected blood, and a few cases of HIV-2 have been identified in US residents. Both of these viruses can cause acquired immunodeficiency syndrome, or AIDS.
Antibodies to Human T-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 infection can persist for a lifetime, but rarely causes major illnesses in most people who are infected. In rare instances, the virus may, after many years of infection, 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. Disease associations with HTLV-II have been hard to confirm, but the virus may cause subtle abnormalities of immunity that lead to frequent infections, or rare cases of neurological disease.
This test is done to detect evidence of infection with the 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 (no cases have been recognized in this country for many years) because the infection is very rare in blood donors, and because the spirochete is fragile and unlikely to survive blood storage conditions.
Nucleic Acid Amplification Testing (NAT)
NAT employs testing technology that directly detects the genetic material of viruses. Because NAT detects a virus’s genetic material — instead of waiting for the body’s response, the formation of antibodies, as with many current tests — it offers the opportunity to reduce the window period during which an infecting agent is undetectable by traditional tests, thus further improving blood safety.
NAT is being used to detect HIV-1 and HCV, and this technology is under investigation for detecting other infectious disease agents.
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 when the donor was 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 and the donor is notified. The donor’s name is then added to a donor deferral list and is prohibited from donating blood indefinitely.