CSMLS - Canadian Society for Medical Laboratory Science Transfusion Science: ABO Discrepancies Questions and Answers — Questions and Answers
Question 1: A patient's forward grouping shows agglutination with Anti-A and weak (1+) agglutination with Anti-B. The reverse grouping shows agglutination with B cells only. The patient's diagnosis is a severe gastrointestinal infection caused by a gram-negative bacteria. What is the MOST likely cause of this ABO discrepancy?
- Acquired B antigen (Correct answer)
- A subgroup with anti-B alloantibody
- Recent transfusion of group B red cells
- Cold autoantibody interference
Correct answer: Acquired B antigen
The 'Acquired B' phenomenon occurs in group A individuals, often with gastrointestinal diseases or gram-negative septicemia. Bacterial enzymes modify the A antigen (N-acetyl-D-galactosamine) to a structure that resembles the B antigen (D-galactose), causing a weak reaction with some anti-B reagents. [2, 6, 8] The patient's own anti-B does not react with this 'B-like' antigen, so the reverse group is consistent with a group A individual. [6, 8]
Question 2: A technologist performs ABO grouping on a 92-year-old patient. The results are: Anti-A: 0, Anti-B: 4+, A1 Cells: 0, B Cells: 1+. Which of the following is the most appropriate first step to resolve this discrepancy?
- Perform an antibody screen to identify the unexpected antibody.
- Use Dolichos biflorus lectin to test for A1 subgroup.
- Incubate the patient's plasma with reagent cells at room temperature or 4°C. (Correct answer)
- Wash the patient's red cells with warm saline and repeat the forward grouping.
Correct answer: Incubate the patient's plasma with reagent cells at room temperature or 4°C.
This is a Group 1 discrepancy, characterized by weak or missing antibodies in the reverse grouping. [4] This is common in elderly patients due to decreased antibody production. [4] To enhance the weak reaction, incubating the patient's plasma with the reagent A1 and B cells at lower temperatures (room temperature or 4°C) can help facilitate antibody binding and resolve the discrepancy. [2, 4]
Question 3: A patient's ABO typing results are as follows: Anti-A: 4+, Anti-B: 4+, A1 Cells: 2+, B Cells: 2+. All tubes, including the autocontrol, show agglutination at the immediate spin phase. Microscopic examination reveals rouleaux. What is the best technique to resolve this discrepancy?
- Perform a Direct Antiglobulin Test (DAT).
- Adsorb the cold autoantibody from the plasma.
- Perform saline replacement on the reverse grouping tubes. (Correct answer)
- Test the patient's cells with anti-A1 lectin.
Correct answer: Perform saline replacement on the reverse grouping tubes.
This is a classic presentation of a Group 3 discrepancy caused by rouleaux formation, often due to abnormal plasma proteins (e.g., in multiple myeloma). Rouleaux causes non-specific aggregation of red cells. The saline replacement technique resolves this by dispersing the rouleaux; the technologist removes the patient's plasma and replaces it with saline. True agglutination will remain, while rouleaux will disperse. [2, 5, 9]
Question 4: A discrepancy is observed where the forward grouping suggests Group O (No agglutination with Anti-A or Anti-B), but the reverse grouping suggests Group A (agglutination with B cells only). The patient has a history of leukemia. What is the most likely cause of this discrepancy?
- Presence of a potent cold autoantibody.
- Weakened A antigen expression due to malignancy. (Correct answer)
- Transfusion of non-group specific plasma.
- Chimerism from a recent stem cell transplant.
Correct answer: Weakened A antigen expression due to malignancy.
This is a Group 2 discrepancy, characterized by weak or missing antigens in the forward grouping. [4] Certain diseases, particularly hematologic malignancies like leukemia, can suppress the expression of ABO antigens on red blood cells, leading to weaker-than-expected or negative reactions in the forward type. [3, 4] The reverse grouping, which reflects the patient's true ABO antibodies, remains as expected for their actual blood group.
Question 5: Which of the following scenarios would most likely result in a mixed-field reaction in the forward ABO grouping?
- A Group A patient with hypogammaglobulinemia.
- A Group O patient with a strong cold autoantibody.
- A Group B patient recently transfused with Group O red blood cells. (Correct answer)
- A Group AB patient with multiple myeloma.
Correct answer: A Group B patient recently transfused with Group O red blood cells.
A mixed-field reaction shows two distinct cell populations: one agglutinated and one unagglutinated. This is a hallmark of a recent transfusion with a different, compatible ABO group. A Group B patient receiving Group O red cells will have both their own B-antigen-positive cells (which will agglutinate with Anti-B) and the transfused O cells (which will not), creating a mixed-field appearance. [3, 4, 11]
Question 6: The forward type of a patient is Group A, but the reverse type shows an unexpected reaction with A1 cells, suggesting Group O. The patient is a healthy 25-year-old blood donor. What is the most probable cause for this discrepancy?
- The patient has the Bombay phenotype (Oh).
- The patient is an A2 subgroup with anti-A1. (Correct answer)
- The patient has an acquired B antigen.
- There is rouleaux formation interfering with the test.
Correct answer: The patient is an A2 subgroup with anti-A1.
This discrepancy pattern is characteristic of an A2 individual who has produced an anti-A1 antibody. The patient's red cells are group A (specifically, A2 subgroup) and react with Anti-A. However, their plasma contains the expected anti-B and also an unexpected anti-A1, which reacts with the A1 cells used in the reverse grouping panel. This causes the reverse group to appear like that of a Group O individual. [2, 4, 11]
CSMLS - Canadian Society for Medical Laboratory Science Transfusion Science: ABO Discrepancies Questions and Answers
A patient's forward grouping shows agglutination with Anti-A and weak (1+) agglutination with Anti-B.
The reverse grouping shows agglutination with B cells only.
The patient's diagnosis is a severe gastrointestinal infection caused by a gram-negative bacteria.
What is the MOST likely cause of this ABO discrepancy?
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