Blood typing is performed to determine which kind of antigens (sometimes known as "markers") are on the surface of red blood cells. Typing is important because in some instances a person's serum (the fluid, non-cellular portion of the blood) contains antibodies against the markers not represented on that person's cells.
All pregnant women and all those who need a transfusion need this test. All donated blood has this blood type test performed on it so that donor and recipient blood types are compatible.
Paternity testing often involves blood typing, which may or may not be able to prove if someone could be the parent of a child.
There are other, less common indications for blood typing, such as genetic studies.
A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or "tourniquet," is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them.
A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial for testing in the laboratory.
After the needle is withdrawn, the puncture site is covered with a bandage for a short time to prevent bleeding. Tests are performed in the laboratory on the red cells in the sample to determine both the Rh and ABO types..
A person should request specific instructions from his or her doctor. Generally, no special preparation is required.
For blood transfusion:Ideally, a person should be transfused only with blood of his or her own ABO and Rh type. This eliminates the possibility that serum will attack cells in either direction. Today's blood banking systems are large enough that an exact match is nearly always possible.
However, during severe blood shortages or other emergency situations, it is possible that blood of the correct type might not be available. In these cases, some exceptions to the rule of exact match can be made to work:
When a person needs a transfusion, what he or she needs most is the donated red cells. Essentially everything in the fluid portion of the blood can be supplied in other ways. Thus, it is the match between the donor cells and the recipient serum that cannot be violated. Type O red cells can thus be donated to anyone - because they have no ABO antigens - so long as the red cells are washed in a machine to get rid of donor serum, packed, and administered with IV fluids.
Following the same logic, type A red blood cells can donated to anyone who does not have anti-A antibodies in the serum, namely those with type A or AB blood. Type B cells can go be transfused to persons with blood types B or AB. AB cells would not be washed and packed for this reason because they can go only to type AB persons anyway.
Again by the same principle, Rh negative blood can be transfused to anyone, and Rh positive blood only to Rh positive people. While indeed most Rh positive people do not have anti-Rh antibody, it is still a bad idea to give them Rh positive cells and let them develop the antibodies, because they can cause trouble later, for instance, if the person winds up needing an organ transplant.
In pregnancy:The concern in pregnancy is blood type incompatibility between the mother and the unborn child. As it turns out, ABO incompatibility in pregnancy is a relatively minor issue. One explanation for this is that anti-A and anti-B antibodies have lots of other places to attach in an infant besides the red cells, and thus do little damage.
Very occasionally when a baby of type A or B (or theoretically type AB, in the case of oocyte or embryo donation) is born to a type O mother, enough infant red cells are attacked to cause a high bilirubin level in the infant after birth. This condition can be successfully treated with phototherapy.
Rh incompatibility in pregnancy is a much more significant problem. When an Rh negative mother gives birth to an Rh positive baby, she gets an infusion of infant red cells when the placenta comes loose, causing her to develop anti-Rh antibodies. The trouble happens with the next Rh positive child. The anti-Rh antibodies developed the last time around cross the placenta and attack the unborn child's red cells, leading to severe and sometimes fatal anemia at birth.
This phenomenon can be prevented by giving the mother two shots of anti-Rh antibodies (i.e., RhoGAM), one at week 28 of pregnancy and the other at delivery. These will stop the mother from forming her own anti-Rh antibodies. The anti-Rh given in the shot will disappear in a few months, thus preventing a problem in the next pregnancy.