Donating blood is very safe. The entire process of donating whole blood (that is, blood with all component cells) takes about 1 hour. Blood donors must be at least 17 years old and weigh at least 110 pounds. In addition, they must be in good health: their pulse, blood pressure, and temperature are measured, and a blood sample is tested to check for anemia. They are asked a series of questions about their health, factors that might affect their health, and countries they have visited.

Did You Know... There are very few disorders that disqualify people from giving blood. Most people who are deferred from giving blood are eligible to donate at a later time. Doctors test donated blood for many infectious diseases so the chance a person will get a disease from donated blood is rare.

Conditions that permanently disqualify a person from donating blood include hepatitis B or C, heart disease, certain types of cancer (leukemia, lymphoma, and any type of cancer that has recurred after treatment or that has ever been treated with chemotherapy drugs), severe asthma, bleeding disorders, possible exposure to prion diseases (such as variant Creutzfeldt-Jakob disease—see Prion Diseases: Introduction, AIDS, and possible exposure to the human immunodeficiency virus (HIV, the virus that causes AIDS) due to high-risk behaviors (see Human Immunodeficiency Virus (HIV) Infection). Conditions that temporarily disqualify a person include malaria (if it has been less than 3 years since the person last experienced symptoms), cancer that has been treated with surgery or radiation (if it has been less than 5 years since the person last received treatment), pregnancy, recent major surgery, poorly controlled high blood pressure, low blood pressure, anemia, the use of certain drugs, exposure to some forms of hepatitis, and a recent blood transfusion.

Generally, donors are not allowed to give blood more than once every 56 days. The practice of paying donors for blood has almost disappeared, because it encouraged needy people to present themselves as donors and then sometimes to deny having any conditions that would disqualify them.

Blood Typing Blood is classified by type. A person's blood type is determined by the presence or absence of certain proteins (Rh factor and blood group antigens A and B) on the surface of red blood cells. The four main blood types are A, B, AB, and O, and for each type, the blood is either Rh-positive or Rh-negative. For example, a person with O-negative blood has red blood cells that lack both A and B antigens and the Rh factor. A person with AB-positive blood has red blood cells that have A and B antigens and the Rh factor. Some blood types are far more common than others. The most common blood types in the United States are O-positive and A-positive, followed by B-positive, O-negative, A-negative, AB-positive, B-negative, and AB-negative. A blood transfusion is safest when the blood type of the transfused blood precisely matches the recipient's blood type. Therefore, before a transfusion, blood banks perform a test called a “type and cross-match” on the donor's and the recipient's blood. This test minimizes the chance of a dangerous or possibly fatal reaction. However, in an emergency, anyone can receive type O red blood cells. Thus, people with type O blood are known as universal donors. People with type AB blood can receive red blood cells from any blood type and are thus known as universal recipients. Recipients whose blood is Rh-negative must receive blood from Rh-negative donors, but recipients whose blood is Rh-positive may receive Rh-positive or Rh-negative blood.

A person who is deemed eligible to donate blood sits in a reclining chair or lies on a cot. A health care worker examines the inside surface of the person's elbow and determines which vein to use. After the area immediately surrounding the vein is cleaned, a needle is inserted into the vein and temporarily secured with a sterile covering. A stinging sensation is usually felt when the needle is first inserted, but otherwise the procedure is painless. Blood moves through the needle and into a collecting bag. The actual collection of blood takes only about 10 minutes.

The standard unit of donated blood is about 1 pint (about 450 milliliters). Freshly collected blood is sealed in plastic bags containing preservatives and an anticlotting compound. A small sample from each donation is tested for the infectious organisms that cause AIDS, viral hepatitis, selected other viral disorders, and syphilis.

Testing Donated Blood for Infections Blood transfusions can transmit infectious organisms carried in the donor's blood. That is why health officials have restricted blood donor eligibility and made blood testing thorough. All blood donations are tested for infection with the organisms that cause viral hepatitis, AIDS, selected other viral disorders (such as West Nile virus), Chagas' disease, and syphilis. Viral Hepatitis Donated blood is tested for infection with the viruses that cause the types of viral hepatitis (types B and C) that are transmitted by blood transfusions. These tests cannot identify all cases of infected blood, but with the rigorous testing and donor screening procedures, a transfusion poses almost no risk of transmitting hepatitis C. The current risk is 1 infection for every 1,500,000 units of blood transfused. Hepatitis B remains the most common potentially serious disorder transmitted by blood transfusions, with a current risk of about 1 infection for every 137,000 units of blood transfused. AIDS In the United States, donated blood is tested for the human immunodeficiency virus (HIV), the cause of AIDS. The test is not 100% accurate, but potential donors are interviewed as part of the screening process. Interviewers ask about risk factors for AIDS—for instance, whether the potential donors or their sex partners have injected drugs or had sex with a man who has male sex partners. Because of the blood test and the screening interview, the risk of contracting HIV infection through a blood transfusion is extremely low—1 in 2,000,000, according to recent estimates. Syphilis Blood transfusions rarely transmit syphilis. Not only are blood donors screened and donations tested for the organism that causes syphilis, but the donated blood is also refrigerated at low temperatures, which kills the infectious organisms.

Last full review/revision July 2007 by Harold S. Kaplan, MD; Donna L. Skerrett, MD