Blood typing
Alternative names:
ABO blood typing; cross matching; Rh typing
How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test:
Adults:
No special preparation is necessary for this test.
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:
While the general physical make-up of people is the same, each individual is unique. Every person has identifiers on cells that allow the body to know that those cells belong to him or her. A and B are common identifiers. Another surface antigen on red blood cells is the Rh factor. Whether this antigen is present or not identifies your blood as Rh+ or Rh.
The ABO typing process has two steps: forward and reverse typing. Initially, your blood is mixed with anti-A serum (serum that contains antibodies against type A blood), then with anti-B serum (serum that contains antibodies against type B blood). A determination of the blood type is based on whether or not the blood clots in the presence of these serums. The second step involves placing your serum (the liquid portion of the blood without the cells) with blood that is known to be type A and type B (AB). With the results of these two tests, your blood type can be determined accurately.
Rh typing is similar to ABO typing. Here, your blood is mixed with serum containing anti-Rh antibodies and then observed for coagulation. If coagulation occurs, then you have Rh-positive blood. If coagulation does not occur, then you have Rh-negative blood.
Blood typing is especially important during pregnancy. If the mother is found to have type O or Rh-negative blood, then the father should also be tested. If the father has type AB or Rh-positive blood, then the mother's blood needs to be checked for Rh antibodies. These antibodies can harm the fetus if not treated (see Rh incompatibility).
|
