Compatibility Testing

The ABO-Rh type, crossmatch, and antibody screen are frequently referred to as compatibility tests (see Chapter 48 ). These tests were designed to demonstrate harmful antigen-antibody interactions in vitro so that harmful in vivo antigen-antibody interactions could be prevented. Donor blood used for emergency transfusion of group-specific blood must be screened for hemolytic anti-A or anti-B antibodies, or both. All donor blood must be tested for the correct ABO and Rh type and screened for unexpected antibodies. Similarly, recipient blood must also undergo ABO-Rh typing, as well as testing for unexpected antibodies. Once this has been completed, proper selection of donor blood requires a test for compatibility between recipient blood and donor blood; this test is known as a crossmatch ( Fig. 47-1 ).

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Figure 47-1 Outline of the tests used for a crossmatch. The x over the word crossmatch means that the crossmatch is not included in the type and screen.

ABO-Rh Typing

Determination of the patient's correct blood type is exceedingly important because the most serious and tragic reactions are usually caused by accidental transfusion of ABO-incompatible blood. These reactions result from naturally occurring antibodies (i.e., anti-A and anti-B), which activate complement and lead to rapid intravenous hemolysis. Anti-A or anti-B antibodies, or both, are formed whenever the individual lacks either or both of the A and B antigens. In essence, antibodies are directed against those antigens that are lacking in the individual's own cells. ABO typing is performed by testing RBCs for the A and B antigens and the serum for the A and B antibodies before transfusion ( Table 47-2 ).

The only additional required testing is that for the Rh(D) antigen. Antigen D is a very common one and, except for the A and B antigens, the one most likely to

TABLE 47-2 -- ABO compatibility testing

	Red Cells Tested With		Serum Tested With
Blood Group	Anti-A	Anti-B	A Cells	B Cells
A	+	-	-	+
B	-	+	+	-
AB	+	+	-	-
O	-	-	+	+

produce immunization. Approximately 60% to 70% of Rh(D)-negative recipients are immunized (produce anti-D) if they are given blood transfusions with Rh(D)-positive blood. About 85% of individuals possess the D antigen and are classified as Rh(D) positive; the remaining 15%, who lack the D antigen, are classified as Rh(D) negative. Because anesthesiologists and surgeons often have difficulty understanding the blood grouping system, Table 47-3 is included to facilitate identification of donor blood groups whose blood patients can receive.

Crossmatching

A crossmatch is essentially a trial transfusion within a test tube in which donor RBCs are mixed with recipient serum to detect a potential for serious transfusion reaction. The crossmatch can be completed in approximately 45 to 60 minutes and is carried out in three phrases: an immediate phase, an incubation phase, and an antiglobulin phase.

The first phase is conducted at room temperature and is a check against errors in ABO typing. It detects ABO incompatibilities and those caused by naturally occurring antibodies in the MN, P, and Lewis systems. This takes approximately 1 to 5 minutes to complete.

The second phase involves incubation of the first-phase reactions at 37°C in albumin or low-ionic strength salt solution. The addition of albumin and low-ionic-strength salt solution aids in the detection of incomplete antibodies or those antibodies that are able to attach to a specific antigen (i.e., sensitization) but are unable to cause agglutination in a saline suspension of RBCs. This phase primarily detects antibodies in the Rh system. The incubation of 30 to 45 minutes in albumin and of 10 to 20 minutes in low-ionic-strength salt solution in this phase is of sufficient duration to allow antibody uptake sensitization by cells so that incomplete antibodies missed in this phase can be detected in the subsequent antiglobulin phase.

The third phase of the crossmatch, the indirect antiglobulin test, involves the addition of antiglobulin sera to the incubated test tubes. With this addition, antihuman antibodies present in the sera become attached to the antibody globulin on the RBCs, causing agglutination. This antiglobulin phase detects most incomplete antibodies in the blood group systems, including the Rh, Kell, Kidd, and Duffy blood group systems.

Although all three phases of the crossmatch are important, the first two stages are of prime importance in preventing serious hemolytic transfusion reactions (see "Type and Screen"). The incubation and antiglobulin phases are especially important because the antibodies appearing in

TABLE 47-3 -- Donor blood groups that patients can receive

Donor	Recipient
O	O, A, B, AB
A	A, AB
B	B, AB
AB	AB

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Antibody Screening

The antibody screen is also carried out in three phases and is similar in length to the crossmatch. The screen, however, is a trial transfusion between the recipient's serum and commercially supplied RBCs that are specifically selected to contain optimal numbers of RBC antigens, or those antigens that will react with antibodies that are commonly implicated in hemolytic transfusion reactions.

The screen for unexpected antibodies is also used on donor serum and is performed shortly after withdrawal of blood from the donor. It is necessary to screen donor serum for unexpected antibodies to prevent their introduction into the recipient serum. This screen is performed primarily to prevent reactions between transfused donor units.