Blood typing is a screening test to determine blood groups and Rh antigen for blood transfusion and pregnancy. The four blood groups A, B, O, and AB are determined by the presence of antigens A and B or their absence (O) on a patient’s red blood cells. In addition to ABO grouping, most immunohematology testing includes evaluation of Rh typing tests for Rh(D) antigen. Blood cells that express Rh(D) antigen are Rh positive. Red blood cells found lacking Rh(D) are considered Rh negative. Rh typing is also important during pregnancy because of the potential for mother and fetus Rh incompatiblity. If the mother is Rh negative but the father is Rh positive, the fetus may be positive for the Rh antigen. As a result, the mother’s body could develop antibodies against the Rh antigen. These antibodies may cross the placenta and cause destruction of the baby’s red blood cells, resulting in a condition known as hemolytic disease of the fetus and newborn.
Blood typing is performed by agglutination testing. The patient’s red cells are tested with anti-A and anti-B antibodies for the presence or absence of agglutination (forward type, aka cell type), and patient’s serum or plasma is tested against known A and B cells (reverse type, aka serum type, aka back type). Rh typing is done by testing patient red blood cells with anti-D antibody.
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Transfusion of blood components of the correct blood type is necessary in order to prevent an adverse immunologic reaction. These reactions can range from very mild and sub-clinical to very severe or fatal, depending upon the components involved and condition of the recipient. Therefore, accurate assessment of both blood component and recipient ABO and Rh status is mandatory. The results of this testing will determine what blood group types a recipient may receive safely. For plasma components such as fresh frozen plasma (FFP) and platelets, it is important that the plasma be compatible with the recipient’s red blood cells. This is always true for FFP which must be transfused in adequate volume to replace essential components in the recipient. For platelets, they can be concentrated if the ABO types are incompatible such that the amount of plasma given to the recipient is reduced to a minimum and the resulting hemolysis, if any, is reduced accordingly.
Utility
- Determination of patient’s blood group before transfusion or transplantation
- Prenatal screening for fetal maternal incompatibility that might cause hemolytic disease of the newborn
- Blood typing for other medical and investigation reasons
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