Normal findings

Compatibility

Test explanation and related physiology

With blood typing, ABO and Rh antigens can be detected in the blood of prospective blood donors and potential blood recipients. This test is also used to determine the blood type of expectant mothers and newborns. A description of the ABO system, Rh factors, and blood crossmatching is reviewed here.

ABO system

Human blood is grouped according to the presence or absence of A or B antigens. The surface membranes of group A red blood cells (RBCs) contain A antigens; group B RBCs contain B antigens; group AB RBCs have both A and B antigens; and group O RBCs have neither A nor B antigens. In general, a person’s serum does not contain antibodies to match the surface antigen on their RBCs. That is, persons with group A antigens (type A blood) will not have anti-A antibodies; however, they will have anti-B antibodies. The converse is true for persons with group B antigens. Group O blood will have both anti-A and anti-B antibodies (Table 1). These antibodies against A and B blood group antigens are formed in the first 3 months of life after exposure to similar antigens on the surface of naturally occurring bacteria in the intestine.

Table1. Blood typing

Blood transfusions are actually transplantations of tissue (blood) from one person to another. It is important that the recipient not have antibodies to the donor’s RBCs. If this were to occur, there could be a hypersensitivity reaction, which can vary from mild fever to anaphylaxis with severe intravascular hemolysis. If donor ABO antibodies are present against the recipient antigens, usually only minimal reactions occur.

Persons with group O blood are considered universal donors because they do not have antigens on their RBCs. People with group AB blood are considered universal recipients because they have no antibodies to react to the transfused blood. Group O blood is usually transfused in emergent situations in which rapid, life-threatening blood loss occurs and immediate transfusion is required. The chance of a transfusion reaction is least when type O is used. Women of childbearing potential should receive group O-negative blood, and men generally receive group O-positive blood when emergency transfusion before type-specific or cross matched blood is required.

ABO typing is not required for autotransfusions (blood donated by a patient several weeks before a major operation and then transfused postoperatively). However, in most hospitals, ABO typing is performed on those patients in the event that further blood transfusion of banked blood is required.

Rh factor

 The presence or absence of Rh antigens on the RBC’s surface determines the classification of Rh positive or Rh negative. After ABO compatibility, Rh factor is the next most important antigen affecting the success of a blood transfusion. The major Rh fac tor is Rh_o (D). There are several minor Rh factors. If Rh_o (D) is absent, the minor Rh antigens are tested. If negative, the patient is considered Rh negative (Rh^–).

Rh– persons may develop antibodies to Rh antigens if exposed to Rh-positive (Rh+) blood by transfusions or fetal–maternal blood mixing. All women who are pregnant should have a blood typing and Rh factor determination. If the mother’s blood is Rh–, the father’s blood should also be typed. If his blood is Rh+, the woman’s blood should be examined for the presence of Rh antibodies (by the indirect Coombs test). Hemolytic disease of the newborn can be prevented by Rh typing during pregnancy. If the mother is Rh–, she should be advised that she is a candidate for RhoGAM (Rh immunoglobulin that “neutralizes” the Rh antigen) after the birth. RhoGAM can reduce the chance of fetal hemolytic problems during subsequent pregnancies.

Other blood typing systems

 There are nine different gene codes for blood groups assayed. Most are minor and not clinically significant. However, in certain clinical circumstances, these minor blood group antigens and acquired antigens can become significant. This may occur with frequent blood transfusions or in patients with leukemia or lymphoma.

Blood crossmatching

 Although typing for the major ABO and Rh antigens is no guarantee that a reaction will not occur, it does greatly reduce the possibility of such a reaction. Many potential minor antigens are not routinely detected during blood typing. If allowed to go unrecognized, these minor antigens also can initiate a blood transfusion reaction. Therefore blood is not only typed but also crossmatched to identify a mismatch of blood caused by minor antigens. Crossmatching always includes an indirect Coombs test. Only blood products containing RBCs need to be cross matched. Plasma products do not need to be crossmatched but should be ABO compatible because other cells (WBCs and platelets) have ABO antigens.

Homologous (donor and recipient are different people) and directed (recipient chooses the donor) blood for donation must be rigorously tested before transfusion. Autologous (recipient and donor is the same person) blood for transfusions, however, is not subject to that same testing.

Finally, one must be aware of graft-versus-host disease (GVHD) in which donor lymphocytes included in the blood transfusion may engraft and multiply in the recipient.

Procedure and patient care

 • See inside front cover for Routine Blood Testing.

 • Fasting: yes

• Blood tube commonly used: red. Verify with laboratory.

Abnormal findings

See Test explanation and related physiology section