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Blood Basics > The Blood Supply

Testing Performed on Donated Blood


Blood is tested to determine the donor's ABO group (A, B, O, AB) and Rh type (positive or negative). This is critical in selecting compatible blood for a patient in need of a transfusion.


All donors are tested to determine if their plasma contains unexpected antibodies to red blood cell antigens (genetic marker). These antibodies, if not identified, can cause problems in a blood recipient.


Screening donated blood for infectious diseases that can be transmitted through blood transfusion is very important in ensuring safety. A positive screening test in any of the following tests for infectious disease is followed by a confirmatory test, since it is possible to have false positive test results. A false positive occurs when the screening test is positive but it cannot be confirmed. This means a donor was not exposed to the infectious agent being tested for but the screening test was positive. The following eight tests to be performed on each unit of donated blood are required by the Food and Drug Administration (FDA):

Hepatitis B Surface Antigen (HBsAg) and Core Antibody (anti-HBc)

The hepatitis B virus has an inner core and an outer envelope (the surface). The HBsAg test detects the outer envelope or surface of the virus.

The anti-HBc tests for the presence of antibodies to the inner core of the hepatitis B virus. In the absence of any other positive test for hepatitis B, the presence of antibodies to HBc suggests the individual may have had past exposure to the hepatitis B virus and it is unlikely he/she is infectious.

Hepatitis C Virus Antibody (anti-HCV) and Nucleic Acid Test (NAT)

Two tests are done to detect hepatitis C infection. The anti-HCV test detects antibodies to the hepatitis C virus. A positive result suggests the donor has been exposed to the hepatitis C virus and may be infectious. The second test detects RNA, the genetic material, of the hepatitis C virus.

Alanine-Aminotransferase Test (ALT)

The ALT measures the level of the liver enzyme (ALT) that is found in the blood. An elevated result suggests the possibility of abnormal liver function, but this may not necessarily be related to viral hepatitis. This test was initiated prior to more sophisticated tests performed today.

Human T-Lymphotrophic Virus Type I and Type II (HTLV I/II) Antibody

HTLV I/II testing detects antibody to the HTLV-I and HTLV-II viruses. These viruses are uncommon in the United States. HTLV-I has been associated with adult T-cell leukemia/lymphoma (ATL) and HTLV-I is associated with myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-II has not been clearly associated with any diseases, but may lead to subtle abnormalities of the immune system.

Syphilis Antibody

Syphilis tests detect the presence of an antibody to the organism "Treponema pallidum" that causes syphilis. This test has been performed on blood donors since shortly after World War II when the rate of infection was much higher. The risk of transmitting syphilis through a blood transfusion today is very small, since the rate of infection is low in blood donors and the organism causing syphilis is very fragile and unlikely to survive blood storage.

Human Immunodeficiency Virus 2 (HIV-1/2) Combo Antibody and Nucleic Acid Amplification Testing (NAT)

HIV-1 and/or HIV-2 virus cause acquired immunodeficiency syndrome, or AIDS. HIV-1 is more common in the United States while HIV-2 is prevalent in Western Africa. Two tests are done for detecting HIV. One screens for antibodies to both HIV-1 and HIV-2 viruses. The other detects RNA, the genetic material, of the HIV-1 virus.

-- Susan T. Johnson, MSTM, MT(ASCP)SBB
Manager, Immunohematology Services
The Blood Center of Southeastern Wisconsin, Inc.