Welcome to Flight School
By Ashley Smith
The convention center was transformed to “Fighter Town USA” on Sunday as “Maverick” — a.k.a. Martha Rae Combs, MT(ASCP)SBB, from Duke University Medical Center — and “Goose” — a.k.a. Susan T. Johnson, MSTM, MT(ASCP)SBB, from BloodCenter of Wisconsin — tested the serology flight skills of attendees in session 5209-TC, “Top Gun Serology Flight School.”
Transfusion service staff often are faced with difficult decisions when matching blood for patients, and this process begins with determining what steps to take first in evaluating the antibody identification panel results. Although the “crossing-out” method takes more time, it is the best method to begin with because it reduces bias. Also, it must be performed to exclude the presence of other alloantibodies and ensure the correct specificity. Matching a pattern is acceptable, but this cannot be the only method of identification because, as Johnson stated, “some things can be missed if this is the only method used.” It can be useful because it is quick and provides clues about the type of additional cells to select.
The AABB Standards for Immunohematology Reference Laboratories defines the requirements for detecting antibodies, but the type and number of antigen-positive cells needed to rule out the presence of antibodies is still not specified. In addition, transfusion services are not required to follow these standards, but as Johnson pointed out, “it is good information to know.” Once the antibodies have been determined, antigen-negative blood can be found by antigen typing the units, but the cost of antisera must be considered. “Cost has increased over the past few years, so it is important to note how many units must be screened,” she said. “It may be more economical to crossmatch first and then type.”
Once antibodies have been identified, there are different pretransfusion testing options to avoid repeat testing. “There has been a change over the years as more people are now considering whether a full panel needs to be repeated,” explained Johnson. According to Standard 5.13.3.3 (AABB Standards for Blood Banks and Transfusion Services or BBTS Standards), “methods of testing shall be those that identify additional clinically significant antibodies.” A selected cell panel can be used to rule out additional antibodies, but it is not necessary to confirm previously identified antibodies. Johnson indicated this standard also can be interpreted in a different way. “More people are following the idea that if the reactivity seen on the antibody detection test fits the pattern for previously identified antibodies, it is acceptable to give antigen-negative crossmatch-compatible blood.”
There are multiple methods for routine antibody detection including gel, the polyethylene glycol indirect antiglobulin test (PEG IAT), low-ionic-strength saline (LISS) tube testing and the solid-phase red cell adherence assay (SPRCA), and each has strengths and weaknesses. “Gels are nice because of their specificity, while PEG IAT has an increased sensitivity but may detect unwanted specificities,” explained Combs. “A LISS tube test avoids warm autoantibodies, but it might not be as sensitive as a gel for clinically significant antibodies. The SPRCA has increased sensitivity for both wanted and unwanted specificities,” she continued. The evaluation of alloantibody vs. autoantibody also is important and, as Combs noted, “The most powerful tool we have is to run an autocontrol and then, if it is positive, perform a direct antiglobulin test.”
Phenotyping a patient’s red cells for routine antigens and testing phenotype-matched red cells to distinguish high-incidence antibodies also can be beneficial. “When patients are positive, then it means they have antibodies to high-incidence antigens and may or may not also have antibodies to routine antigens,” said Combs. “But, when patients are negative, it usually indicates they have multiple antibodies to routine antigens.” Performing phenotype testing, however, can be expensive; therefore, it is important to predetermine specific antigens to be included in the testing.
If an antibody test indicates varying levels of reactivity, additional testing methods may be necessary, including tests to determine if the antibodies are high-titer and low-avidity (HTLA) antibodies. Antibody titration is one potential approach but can be a difficult method to use. Another option is testing high-incidence antigen-negative red cells or phenotyping the patient for high-incidence antigens, but with each method, it is difficult to know what to test. “If you are including HTLA specificities, there can be a few problems,” explained Combs. “Older cells and sera are not well characterized for HTLA-like specificities, and the reactions can be weak and inconsistent, making it difficult to get an answer.”
The timing of a weak D test during pretransfusion testing also was included in the flight school curriculum. AABB BBTS Standard 5.20.2 indicates that if a woman’s test results are negative for D antigen, a test for weak D is not required. The test can still be performed, and if it is performed on an obstetric patient, there may be typing discrepancies with laboratories that do not perform the test. If weak D tests are not performed, typing discrepancies among laboratories may still exist and fetal screening surprises may develop.
Combs detailed a case where a 27-year-old female typed D- on a current sample but had a history of typing D+ at immediate spin. Most audience members indicated they would perform a weak D test, while some said they would not but would request a new sample. As Combs told the audience, “it is not a good idea to assume the patient is Rh positive or Rh negative because you can’t rule out a misidentified sample. You also can’t determine an Rh type just on history records.” Because of the discrepancies and uncertainties that still do exist, Combs remarked, “We may eventually have to develop a disclaimer.”
Upon completion of the predetermined curriculum, Combs and Johnson declared the audience “ready for combat and certified in top gun serology.”
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