TESTING OF DONOR
BLOOD FOR INFECTIOUS DISEASES
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:
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 persons 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
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).
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
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.