Blood typing and crossmatching
Blood typing is a laboratory test done to determine a person's blood type based on the proteins on the surface of blood cells. If the person needs a bloodtransfusion, another test called crossmatching is done to find blood from adonor that the person's body will accept. People must receive blood of the same blood type, otherwise, a serious, even fatal, transfusion reaction can occur.
A child inherits genes from each parent that determine his blood type. This makes blood typing useful in paternity testing. Legal investigations may require typing of blood to identify persons involved in crimes or other legal matters.
Blood typing and crossmatching tests are performed in a blood bank laboratoryby technologists trained in blood bank and transfusion services. The tests are done on blood after it has been separated into cells and serum. Serum is the yellow liquid left after the blood clots.
Blood typing and crossmatching tests are based on the reaction between antigens and antibodies. An antigen is a foreign substance that causes the body tolaunch an attack, known as an immune response, against it. The body builds proteins called antibodies to neutralize each specific antigen.
The antigens found on the surface of red blood cells determine a person's blood type. When red blood cells having a certain blood type antigen are mixed with serum containing antibodies against that antigen, the antibodies attack and stick to the antigen. In a test tube, this reaction is observed as the formation of clumps of cells (clumping).
When blood is typed, a person's blood is mixed in a test tube with commercially-prepared serum and cells. Clumping tells which antigens are present and reveals the person's blood type. When blood is crossmatched, patient serum is mixed with cells from donated blood that might be used for transfusion. Clumping or lack of clumping in the test tube tells whether or not the blood is compatible.
Although there are over 600 known red blood cell antigens organized into 22 blood group systems, routine blood typing and crossmatching is usually concerned with only two systems: the ABO and Rh blood group systems.
A person's ABO blood type--A, B, AB, or O--is based on the presence or absence of the A and B antigens on his red blood cells. The A blood type has only the A antigen, and the B blood type has only the B antigen. The AB blood typehas both A and B antigens, and the O blood type has neither A nor B antigen.Although the distribution of each of the four ABO blood types varies among racial groups, O is the most common and AB is the least common. A person must receive ABO-matched blood for a transfusion. ABO incompatibilities are the major cause of fatal transfusion reactions.
The Rh, or Rhesus system, is also examined during crossmatching. Although there are more than 50 Rh antigens, in routine blood typing and crossmatching tests, only the D antigen, also known as the Rh factor or Rho[D], istested for. If the D antigen is present, the person is Rh-positive; if the Dantigen is absent, that person is Rh-negative. In transfusions, the Rh system is next in importance after the ABO system.
Rh incompatibility is the most common and severe cause of hemolytic disease of the newborn (HDN), a type of anemia also known as erythroblastosis fetalis.This incompatibility can occur when an Rh-negative woman and an Rh-positiveman produce an Rh-positive baby. Cells from the baby can cross the placenta and enter the mother's bloodstream, causing the mother to make anti-D antibodies. These antibodies re-cross the placenta to destroy the baby's red blood cells, causing severe or fatal anemia. This condition can be prevented throughinjections of a specific immunoglobulin protein if it is recognized early inpregnancy.
When a transfusion is needed, blood from a donor and the recipient are typedand crossmatched. In a test tube, serum from the patient is mixed with red blood cells from the potential donor. If clumping occurs, the blood is not compatible; if clumping does not occur, the blood is compatible. If an unexpectedantibody is found in either the patient or the donor, the blood bank does compatibility testing procedures are designed to provide the safest blood product possible for the recipient.
In an emergency, when there is not enough time for blood typing and crossmatching, O red blood cells may be given, preferably Rh-negative. O blood type iscalled the universal donor because it has no ABO antigens for a patient's antibodies to attack.
To collect the 10 mL blood needed for these tests, a healthcare worker ties atourniquet above the patient's elbow, locates a vein in the inner elbow region, and inserts a needle into that vein. Vacuum action draws the blood through the needle into an attached tube. Collecting the sample takes only a few minutes. Blood typing and crossmatching must be done three days or less beforea transfusion.