Under the common ABO blood typing process, a person's blood type could be A, B, AB or O. It is very important to know your own blood type, as well as the blood type of your spouse and children. Important decisions concerning emergency blood transfusions may have to made quickly, so having this information on hand for medical professionals can shave precious minutes off the initial triage process following a trauma. A person's blood type is determined largely by genetics, and does not change through his or her lifetime. A simple blood typing test can be performed literally anywhere by anyone through the use of specially-treated testing cards.

One of the main factors which determine blood type is family genetics. A child receives separate sources of genetic code called alleles from each parent at the time of conception. One of the alleles located on chromosome 9 contains the precise blood type of the donor parent, and is classified as A, B, AB or O. An additional factor is called the Rhesus factor, which could be positive or negative. The actual blood type of a child is determined by the dominant blood type between the two parents. A and B are both dominant over O, which means a child that receives an A blood type from the father and an O blood type from the mother will have an A blood type.

Subsequently, A and B are considered to be codominant, which means a child inheriting an A blood type from the mother and a B blood type from the father will most likely have an AB blood type. Only two recessive O blood type genes from both parents will result in a child having an O blood type. An O negative blood type is considered to be a universal donor, since it contains nothing which would appear foreign to someone else's blood type. Those with A or B positive blood types must not receive blood infusions of the opposite type, since the body's natural defenses will attack the incoming blood cells as they would any other infection.

A person's blood type is determined through a simple ABO test available at a doctor's office, blood donation center or even through pharmacies. A drop of blood is placed on two separate testing circles marked A and B. The card has already been prepared with dried serum containing anti-A and anti-B chemicals. If the blood reacts to the A circle but not the B circle, then the tester's blood type is considered to be A. A reaction to both circles indicates an AB blood type, while a complete non-reaction to either circle indicates an O blood type. The reaction is caused by the chemicals on the card coming in contact with type A or type B antigens on the surface of the red blood cells. Anti-A reacts with an A blood type and Anti-B reacts with a B blood type.