The decision to transfuse blood must be conveyed to a blood bank as a written or electronic order by a physician. In emergent situations, a verbal request for blood may be appropriate, but this should be documented and followed by a written or electronic order as soon as possible. In order to provide compatible blood to a patient, a pretransfusion sample is required for blood bank testing. The sample must be labeled with the patient’s name as well as a second unique identifier such as the patient’s date of birth (DOB) or a medical record number. Most institutions also require additional information on the specimen such as
the identity of the phlebotomist and the date that the sample was drawn. Due to the potentially lethal consequences of incorrect patient identification during sample acquisition or errors during testing, most institutions require two blood bank specimens to be drawn and tested independently from a patient (preferably at different times). Additionally, the results of any testing performed should always be compared to any historical blood bank data available for the patient.
Pretransfusion testing needs to be performed using the patient’s red blood cells (RBC
) and either plasma or serum. Usually, hemolyzed or lipemic samples are not accepted by blood banks as testing such samples may yield inaccurate results. Once an appropriate specimen is received, testing is performed to determine the patient’s ABO group and Rh (D) type followed by screening of the patient’s plasma for unexpected red cell antibodies. After the testing is completed, a suitable unit of donor blood is selected for the patient and checked for compatibility by performing a crossmatch.
AGGLUTINATION (DAT AND IAT)
The majority of testing performed in blood banks involves looking for the presence or absence of agglutination using patient, donor, or reagent red cells and patient plasma or reagent antibodies. The goal of this testing is to predict compatibility of blood products upon transfusion. Agglutination is “clumping” of red cells caused by the binding of antibody to antigens on the red cell membrane. This usually occurs in two phases: (1) sensitization of the red cells by the antibody binding to antigens and (2) lattice formation that results in macroscopic agglutination. Centrifugation is usually necessary to bring red cells in close proximity for agglutination as there is a net negative charge on the surface of red cells that prevents their aggregation and agglutination. As it is possible for IgG
antibodies to result in red cell sensitization without agglutination, it is often necessary to add a secondary antibody to cause lattice formation. The secondary antibody required is an antibody to human globulins, specifically IgG
and complement. This anti-human globulin (AHG
) or Coombs reagent can be used to perform a direct antiglobulin test (DAT
) as well as an indirect antiglobulin (IAT
). When performing a DAT
is added to a patient’s RBCs that are suspected to be sensitized. This is unlike an IAT
is added to a suspension of reagent (or donor) red cells and the patient’s plasma that is suspected to contain an antibody to the reagent (or donor) red cells. In either test, the addition of AHG
will result in agglutination if the red cells are sensitized by the antibody. Although many institutions perform pretransfusion testing as described above in small test tubes, some larger institutions are performing some (or most) of their testing using newer technology (gel columns and solid-phase testing).
PERFORMING A TYPE AND SCREEN
Prior to transfusion of blood, a patient’s ABO and Rh type must be determined, and the patient’s plasma must be checked for expected and unexpected antibodies. The blood typing usually begins with the forward ABO grouping performed by testing a patient’s red cells for the presence of A and B antigens using commercially available anti-A and anti-B antibodies. Subsequently, a
reverse grouping is performed by checking the patient’s plasma for anti-A and anti-B antibodies using reagent group A and group B red cells. With rare exception, patients that lack A or B antigens on their red cells should have an antibody to the antigen that is not present (see Table 17-1
). Any discrepancy in the forward and reverse ABO grouping must be resolved prior to transfusion of blood products, and if urgent transfusion is necessary, group O red cells and group AB plasma should be provided prior to the resolution of this discrepancy.
TABLE 17-1. Antigens and Antibodies Present in Each ABO Blood Group
ABO Blood Group
In order to determine a patient’s blood type, the patient’s red cells must also be tested with anti-D to determine whether the patient expresses the D antigen on his/her red cells. If the patient’s red cells do not agglutinate with the anti-D reagent antibody, some institutions may perform weak D testing by adding AHG
. Weak D testing will detect patients that have a quantitative or qualitative difference in the D antigen expressed on their red cells. Patients that have a weak D phenotype do not have enough D antigen on their red cells to result in direct agglutination, and agglutination is seen only after the addition of AHG
. Patients that have a partial D phenotype have a qualitative difference in the D antigen that also requires AHG
for agglutination. Although some institutions will perform weak D typing on patients, it is not necessary to do so as weak D and partial D patients will otherwise be considered Rh negative and will receive D-negative blood products, which will be compatible with the patient and not result in any harm. The same holds true for pregnant patients; while it is not necessary to perform weak D testing, some institutions choose to do so. If a pregnant patient with a weak D phenotype is typed as D negative (because the institution does not perform weak D testing on patients), she will be treated with Rh immune globulin unnecessarily (as she is not able to be immunized to the D antigen), but this will likely not result in any harm to the patient (also see discussion on prenatal testing in this chapter). However, blood donors must be typed for the weak D antigen because red cells from a weak D donor can immunize Rh-negative patients to the D antigen.
Once the patient’s blood type (ABO and Rh) is determined, the plasma or serum must be checked for unexpected antibodies to non-ABO red cell antigens. The goal of the antibody screen is to detect clinically significant antibodies to red cell antigens that can cause hemolytic transfusion reactions (HTR
) and hemolytic disease of the fetus and newborn (HDFN
). Antibodies that bind to red cells at body temperature (37°C) and are detected using AHG
are more likely to be clinically significant than cold-reactive antibodies that do not result in agglutination at 37°C or the AHG
phase of testing. Some of the more
commonly encountered clinically significant antibodies include antibodies to D, C, E, c, e, S, s, K, k, Fya
, and Jkb
. Reagent red cells must be able to detect these antibodies as well as antibodies to M, N, P1, Lea
, and Leb
After a type and screen is completed, an appropriate blood product can be selected to transfuse the patient. Ideally, ABO-identical blood products should be transfused, but due to inventory management constraints, often, ABO-compatible (but not ABO-identical) products are issued for transfusion. Table 17-2
reviews the type of red cells and plasma that are compatible with each of the ABO blood groups.
Plasma, platelets, and cryoprecipitate can be transfused without a crossmatch. However, for RBC
transfusion, each unit of red cells should be crossmatched with the recipient’s plasma prior to transfusion in order to ensure compatibility. The type of crossmatch necessary depends on whether the patient’s antibody screen is positive or negative.
If the patient’s antibody screen is negative, generally, an immediate spin crossmatch is performed by simply mixing the red cells selected for transfusion with the patient’s plasma and checking for agglutination after centrifugation. An immediate spin crossmatch is essentially a second check of the donor and recipient’s ABO compatibility as agglutination will be seen if the patient’s plasma has ABO antibodies to cognate antigens on the red cells selected for transfusion. In some institutions, patients that have a negative antibody screen may be eligible for an “electronic crossmatch” if at least two verifications of the donor and patient blood type have been performed and if the institution has validated the blood bank information system to not allow an incompatible blood product to be issued to the patient. If the patient has had a positive antibody screen, a full Coombs crossmatch using AHG
must be performed. Many clinically significant non-ABO alloantibodies are IgG
antibodies and will not result in agglutination unless AHG
is added to the suspension of red cells and plasma. Thus, patients that have a positive antibody screen should have the crossmatch performed by incubating the patient’s plasma and donor red cells at body temperature followed by addition of AHG
The sample of blood used to perform the crossmatch (as well as the type and screen) must be recently acquired from the patient if the patient has been transfused or pregnant within the last 90 days. This is necessary because the patient may produce an antibody to an RBC
antigen within as little as a few days after being exposed to allogeneic red cells. Generally, a sample should not
be more than 3 days old if the patient has been recently transfused or pregnant, but a sample drawn up to several weeks prior to transfusion is acceptable if the patient has not had any recent exposure to allogenic blood through transfusion or pregnancy. It is also important to review the blood bank history to prevent the transfusion of RBCs that may have antigens that the patient previously had antibodies to. In these patients, the antibody titer may decrease in strength to below detectable levels, but subsequent exposure to the antigen can result in brisk antibody production and a delayed hemolytic transfusion reaction. Unlike RBC
transfusion, transfusion of other blood products does not require a recent sample because selection is based on the patient’s blood type and a crossmatch does not need to be performed.
TABLE 17-2. Blood Products That are Compatible with Each ABO Blood Group
Compatible Red Cells
A, B, AB, O
O, A, B, AB