The objective of pretransfusion testing is to ensure that enough red blood cells (RBCs) carrying selected red cell components will survive when transfused. Yet, while no one questions the need for subjecting donor and patient blood to pretransfusion testing, some do question how to best and most economically do it. In most blood banks, pretransfusion testing involves determining the ABO and Rh types of both patient and donor blood, screening patient and donor sera for RBC alloantibodies, and performing a major crossmatch (testing the patient's serum against the donor's RBCs). Pretransfusion testing can assure ABO compatibility between donor and patient blood as well as detect most clinically significant RBC alloantibodies that react with antigens on donor RBCs. Unfortunately, it can not always guarantee the normal survival of transfused cells since a minute number of deleterious reactions due to serological incompatibility can still occur.
Over the past 30 years, pretransfusion tests have undergone considerable modification (1-3). In the early 1960s, many blood banks carried out minor crossmatching (testing donor's serum against patient's RBCs) in addition to major crossmatching. It was only in the mid-1970s that the minor crossmatch was finally abandoned as antibody screening of donor blood became routine (4). Most of the early methods for antibody screening and crossmatching involved testing at room temperature. In 1978, the American Association of Blood Banks (AABB) deleted the room-temperature requirement from its Standards (5).
In the past, the prevailing tendency in blood banking was to use to the utmost, and no matter the cost, all available highly sensitive techniques to detect any serological incompatibilities. Yet, as blood-banking and other health care institutions have become more aware of the economy of health care delivery, there has been an increasing trend toward abbreviating the crossmatching procedure in pretransfusion testing (1,2). Obviously, the main reason for abbreviating crossmatching is to save the cost of reagents and labor. The estimated savings from eliminating the anti-human globulin (AHG) phase of a crossmatch is approximately $1.00 in cost and 30% in technologist's time (1-3). With approximately 12,000,000 units of blood transfused annually in the United States, the cost and time savings would be substantial (1-3).
Is crossmatching overdone? Does the cost in time and money outweigh the benefits? Is crossmatching no longer necessary? To help answer such questions, the following review will look at the current state of pretransfusion testing, crossmatching's place in it, and the current practice here at The University of Texas M. D. Anderson Cancer Center.
Blood Sampling and Clerical Checking
In pretransfusion testing, the
importance of careful clerical checking cannot be overemphasized. From the
outset, collecting and properly labeling blood samples from the correct patient
is critical to accurate serological testing and safe blood transfusion. When a
sample is received in the transfusion service laboratory, a medical
technologist must confirm that the information on the label and on the
transfusion request form are identical. The patient's serological and
transfusion history must also be checked and the results of current testing
compared with those of previous tests. In short, any discrepancies must be
resolved before blood can be released for transfusion.
Blood Grouping
The ABO group and Rh type must be determined for the
blood of both the donor and the intended recipient (6). ABO group is determined
by testing the RBCs with anti-A and anti-B reagents and by testing the serum
with A1 and B RBCs. Rh type is determined with anti-Rh (anti-D): if the initial
test of the donor blood with anti-D is negative, then that blood is tested by a
method designed to detect weak D (Du). When either Rh test is positive, the
blood sample label reads "Rh positive." Conversely, when the tests for both D
and Du are negative, the label reads "Rh negative." It is not necessary to
determine whether a patient's RBCs are of the Du type because no harm results
from transfusing Rh-negative blood.
Antibody Detection Tests
Besides clerical checking, grouping, and
typing of donor and patient blood, the serum or plasma of the patient must be
tested against a single-donor suspension of unpooled, group O reagent RBCs.
This meets AABB Standards (6). Such reagent cells are selected because they
carry the blood group antigens necessary for detecting the most important
"clinically significant" RBC alloantibodies, namely, those antibodies reactive
at 37 degrees Celsius and in the AHG phase of crossmatching as opposed to those
reactive at room temperature. In our transfusion service, a set of three
unpooled commercial reagent antibody-screening cells are routinely used to test
for the presence of RBC alloantibodies (7). In brief, three or four drops of
serum and one drop of reagent cells are incubated at 37 degrees Celsius for 30
min, spun, read for hemolysis and agglutination, washed three to four times
with normal saline, and then tested with anti-IgG using an indirect
antiglobulin technique (IAT). The test results are first read macroscopically
for agglutination and then, if negative, microscopically.
The AABB Standards also stipulate that blood from donors with a history of prior transfusion or pregnancy be tested for RBC alloantibodies, preferably at the time of processing. Most blood banks test all donor blood for RBC alloantibodies because of the difficulty in determining donors' past histories. For this test, pooled reagent cells can be used to detect RBC alloantibodies to donor blood (6).
Current antibody screening tests cannot detect all clinically significant antibodies. For instance, antibodies against low-incidence antigens are likely to be missed. Other antibodies manifest a dosage effect dependent on the homozygosity of genes controlling expression of antigens on the reagent screening cells. Examples of such dosage-effect antibodies are anti-Jka, anti-Jkb, anti-Fya, anti-Fyb, anti-C, anti-E, and anti-c. Using three types of screening cells improves the chances of having all major RBC antigens present in their homozygous forms during testing. In cases of positive antibody screening, further serological testing with an expanded panel of reagent RBCs for the identification of clinically significant antibodies is required. Then, once the specificity of the antibody is known, donor units must be screened for the corresponding antigen to select those units that lack the antigen.
Crossmatching Tests
After antibody detection tests are run, a major
crossmatch, namely, testing of the patient's serum with RBCs from the donor,
should be performed before the transfusion of whole blood or RBC components.
For patients who are screened as negative for RBC alloantibodies and who have
no history of such antibodies, only an immediate-spin crossmatch (IS-XM) need
be performed to verify ABO compatibility. IS-XM is performed by mixing two to
four drops of a patient's serum with one drop of donor RBCs suspended in a 2-5%
saline solution in a test tube, centrifuging immediately, and reading for
hemolysis and agglutination. In such cases, the AHG phase of the crossmatch is
not required. Conversely, for patients who are screened as positive for RBC
alloantibodies or who have a history of clinically significant antibodies, an
AHG crossmatch (AHG-XM) must be performed. In AHG-XM, IS-XM is followed by
incubation at 37 degrees Celsius for 30 min and then by IAT using anti-IgG. If
the patient urgently needs a blood transfusion and there is not enough time to
complete a routine study, a shorter incubation time (10 min vs 30 min) can be
achieved using a low-ionic additive solution (7).
The issue of whether to omit AHG-XM for patients screened as negative for RBC alloantibodies remains controversial (1,2). According to a survey conducted by the College of American Pathologists in 1987, only 11% of hospitals at the time routinely used IS-XM without AHG-XM for patients with negative screening for RBC alloantibodies; all others surveyed used AHG-XM (8). Any policy decision by a hospital to omit AHG-XM from crossmatching procedures must be made by a transfusion service's medical director and only after several factors have been considered: (i) the possibility of incompatible crossmatches or hemolytic transfusion reactions due to RBC alloantibodies not detected by antibody screening, (ii) the potential cost- and labor-saving benefits of omitting AHG-XM, and (iii) the sensitivity of the antibody detection test used in the laboratory.
In 1985, we here at M. D. Anderson Cancer Center began to use IS-XM testing without the 37 degrees Celsius-AHG phase for patients screened as negative for RBC alloantibodies. A recent retrospective study of our experience over the ensuing 4 yr (September 1985-August 1989) verified the safety of using IS-XM alone in screening incompatible blood at our institution (9). The study showed that among 92,759 units of packed RBCs transfused, there were only two immediate hemolytic transfusion reactions and eight cases of RBC alloimmunization. The two immediate hemolytic transfusion reactions were caused by transfusion of ABO-incompatible blood resulting from clerical error and would not have been prevented by AHG-XM. All eight cases of RBC alloimmunization were discovered during subsequent routine serological testing prior to additional transfusions to patients. The specificity of the antibodies was anti-Jka in three cases; anti-Fya in two cases; and anti-Jkb, anti-E, and anti-FY3 in one case each. No significant untoward effects of the tranfusions were documented in the records of these eight patients. In effect, the results of our retrospective institutional study agreed with those of previous studies in suggesting a very low risk of transfusing incompatible blood when the 37 degrees Celsius-AHG phase was eliminated from crossmatching procedures (Table 1) (10-12).
| Investigators* | Relative frequency of missing clinically significant antibodies |
|---|---|
| Oberman et al. (1978) | 1/1,744 |
| Mintz et al. (1980) | 1/4,144 |
| Taswell et al. (1981) | 1/4,000 |
| Havemann and Lichtiger (1992) | 1/11,595 |
*See reference list for publication information on each study.
Computerized Crossmatching
In recognition of the improved
capabilities of computer systems, the AABB has stipulated that blood banks may
use computerized crossmatching, instead of serological crossmatching, to detect
ABO incompatibility prior to transfusion and so prevent the release of
ABO-incompatible blood components for transfusion (6). However, the AABB also
stresses that such computerized crossmatching only be done if the following
conditions have been met: (i) The patient's ABO group has been determined twice
(once on a current sample; a second time on the same sample, on a second
current sample, or by comparison with previous records); (ii) the computer
system's database contains the donor unit number, the component name, the ABO
group and Rh type of the component, blood-group confirmatory-test
interpretation and identification, and the ABO group and Rh type of the
patient; (iii) a method is in place to ensure correct entry of data; and (iv)
the system can alert the user to discrepancies between donor unit labeling and
blood-group confirmatory-test interpretation and to ABO incompatibilities
between the patient and donor blood.
"Type and Screen" Policy
In some special instances, crossmatching of
blood is dispensed with according to a policy called "type and screen."
This policy stipulates that blood does not have to be crossmatched in
advance for patients undergoing surgical procedures usually not requiring
blood. The patient's blood is, however, completely tested for ABO group, Rh
type, and RBC alloantibodies and then kept in storage by the transfusion
service in case it is needed for crossmatching (in such a case, typed and
screened patient's blood can be crossmatched by IS-XM and made available in
minutes). With few exceptions, most surgeries at M. D. Anderson Cancer Center
are supported by the "type and screen" policy with no problems. We maintain
near our operating rooms an inventory of blood of all types for surgery
patients. The typed and screened blood samples of patients and the sample
segments from the donor units stored in the surgery inventory are kept in
storage in transfusion service facilities. If a patient in surgery needs blood,
it takes us less than 5 min to complete IS-XM and to deliver blood to the
patient.
Conclusion
Pretransfusion testing is intended to guarantee the normal
survival of transfused RBCs at minimum cost, yet how best to do this remains a
matter of controversy. Because of the increasing awareness of economy and
efficiency in health care delivery, the blood-banking community is now
reexamining its rationale for performing some steps in pretransfusion testing.
While no one questions the importance of careful clerical checks, ABO and Rh
typing, and antibody screening, some do question the extent of crossmatching
procedures. In fact, there has been an increasing trend toward abbreviating the
crossmatching procedure or even eliminating it altogether.
At M. D. Anderson Cancer Center, we currently do and will continue to perform a sensitive antibody screening procedure involving three screening RBC types, a 37 degrees Celsius-AHG phase, and a crossmatch. The type of crossmatch (IS-XM vs AHG-XM) depends on the antibody screening results. In brief, if the antibody screening is negative, then IS-XM is performed prior to release of the blood to ensure ABO compatibility. Conversely, if the antibody screening is positive, then AHG-XM is performed using selected donor units known to be negative for corresponding antigens. At present, the computer system we use in our transfusion service does not meet AABB Standards for computerized crossmatching, and it remains to be seen whether we will adopt computerized crossmatching in the future. So, we continue to depend on IS-XM and AHG-XM to ensure the donor-patient compatibility of transfused blood.
References
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