Transfusion Medicine Case Study: Positive Pretransfusion Test

A 72 year old man was admitted to the hospital for an aneurysm repair. The physician ordered a type and crossmatch for 6 units of blood in preparation for surgery. The patient history included surgery in 2016 during which he was transfused with 4 units of RBCs.

patient’s blood type: A positive

antibody screen: negative

history: anti Jkb  (2016)

6 Jkb negative units were found and full crossmatches were performed. One of the 6 donor units was incompatible. What is the most probable explanation for these findings?

If the patient has a negative antibody screen, and no history of an antibody, most facilities would do an electronic crossmatch or an immediate spin crossmatch. The immediate spin (abbreviated) crossmatch will simply verify ABO compatibility. However, if the patient has a positive antibody screen, we must identify the antibody, phenotype the patient, and do a full AHG crossmatch with donor units that are antigen negative for the corresponding antibody. In this case, the patient had a history of an antibody, so the antibody must be honored, and antigen negative units must be chosen for transfusion.

Kidd antibodies demonstrate dosage, are often weak, and can be found in combination with other antibodies. Because if this, they can be notoriously difficult to detect. They are usually IgG and are made in response to transfusion or pregnancy. Jkb has an antigen frequency of about 73% in the white population and about 43% in the black population. To find antigen negative blood, we consider that about 27% of units would be antigen negative. The tech working on the sample screened 21 units and found 6 that were Jkb negative.

AHG crossmatch results:

unit 1: compatible

unit 2: compatible

unit 3: compatible

unit 4: 3+ at AHG

unit 5: compatible

unit 6: compatible

There are 2 possible scenarios for the above results. A crossmatch is a test between donor’s red blood cells and patient’s plasma. Antigens, we know, are on red blood cells and antibodies are detected in the plasma. So, with a negative antibody screen, crossmatch incompatibility is due either to a patient antibody to a low incidence antigen on the donor red blood cells, or a donor cells with a positive direct antiglobulin test. We can easily rule in or out a positive donor DAT by performing a DAT on the segment. If the donor unit has a positive DAT, the unit should be quarantined and the positive DAT reported to the collecting facility. If the donor unit has a negative DAT, the patient likely has an antibody to a low incidence antigen.

Low frequency antigens are uncommon, but antibodies that recognize them are less rare. Fortunately, for patients with these antibodies to low frequency antigens, finding antigen negative compatible blood is easy. As we can see, 5 of the 6 chosen units were negative for the unknown low frequency antigen and were antiglobulin crossmatch compatible. The low prevalence of the antigen makes compatible blood readily available. If transfusion is necessary, it should not be delayed while waiting for identification of the antibody.

In this case, the antibody screen was repeated and the negative result was verified. In many cases, it may not be possible for a lab to identify the antibody because the lab may not have the necessary panel cells or typing reagents. Yet, these antibodies to low incidence antigens that react at AHG can be clinically significant and cause severe hemolytic transfusion reactions. To identify the antibody, you may need to send the sample to a reference lab for testing against a panel of reagent red cells that express low incidence antigens. Alternately, the donor red cells that were incompatible can be tested against known antibodies to low prevalence antigens  to help identify the antibody.

In this patient, anti-Wra was identified. The incompatible donor unit was verified to be Wra positive. Wra is part of the Diego system, usually IgG, and has ben implicated in hemolytic transfusion reactions.

One of the reasons I have written up this case is questions my Transfusion Medicine students often ask about exam and exam prep questions concerning incompatibility. Below are 2 questions to give examples of the confusion.

“At the indirect antiglobulin phase of testing, there is no agglutination between patient serum and screening cells. One of 3 donor units was incompatible.. The most probable explanation for these findings is that the:

  1. patient has an antibody directed against a high incidence antigen
  2. patient has an antibody directed against a low incidence antigen
  3. donor has an antibody directed against donor cells
  4. donor has a positive antibody screen”5

answer: b

“Which of the following would most likely be responsible for an incompatible antiglobulin crossmatch?

  1. recipient’s red cells possess a low incidence antigen
  2. anti-K antibody in donor serum
  3. recipient’s red cells are polyagglutinable
  4. donor red cells have a positive direct antiglobulin test”4

answer: d

I am asked why is one answer “low prevalence antigen” and one answer “positive DAT”? I typically ask questions of my students to let them reason out the answer. Take a careful look at the words antigen and antibody. Remember that a DAT is a test of red cells, the IAT tests for antibodies in plasma. A crossmatch uses donor red cells against patient plasma. Therefore, even though these are both reasons for the incompatibility of one out of multiple units, each question only has one answer of a common reason for such incompatibility. Be sure to read questions and use your theory and knowledge of testing when encountering discrepancies and problems in Blood Bank. To all of my students: Happy Studying for your ASCP exam!


  1. Fung, Mark K., Technical Manual 18th ed, Bethesda: AABB, 2014.
  2. Harmening, Denise M. Modern Blood Banking and Transfusion Practices, 7th edition, 2019.
  3. Schonewille, Henk, et al. “The importance of antibodies against low‐incidence RBC antigens in complete and abbreviated cross‐matching”. The Journal of AABB. 20 June 2003.
  4. BOC Study Guide, 5th edition. Clinical Laboratory Certification Examinations.  ASCP, 2016
  5. BOC Study Guide, 6th edition. Clinical Laboratory Certification Examinations.  ASCP, 2018

-Becky Socha, MS, MLS(ASCP)CM BB CM graduated from Merrimack College in N. Andover, Massachusetts with a BS in Medical Technology and completed her MS in Clinical Laboratory Sciences at the University of Massachusetts, Lowell. She has worked as a Medical Technologist for over 30 years. She’s worked in all areas of the clinical laboratory, but has a special interest in Hematology and Blood Banking. When she’s not busy being a mad scientist, she can be found outside riding her bicycle.

One thought on “Transfusion Medicine Case Study: Positive Pretransfusion Test”

  1. Thank you for this case study, explanation and practice problems. I ran into a similar question that was phrased something like “If a donor has a positive DAT, in which phase of crossmatching would a positive reaction be observed?”
    a. Immediate spin
    b. AHG phase
    c. Neither
    d. both

    I chose (d) both, but now see correct answer was probably (b) AHG phase. Hopefully that’s right, and thanks again for the blog!

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