Acquired B Phenotype in a Septic Postoperative Patient
Mr. R, 66, is admitted with fever (38.9°C), hypotension (88/54 mmHg), and a distended abdomen three days after a sigmoid colectomy for colon cancer. Blood cultures are pending. The surgeon orders 2 units of RBCs for Hb 7.1 g/dL. In the blood bank, forward typing shows 4+ agglutination with anti-A and 1+ with anti-B; reverse typing shows 4+ anti-A and 2+ anti-B. The patient is group A, Rh-positive by history. A direct antiglobulin test is negative. The technologist calls the OR to ask whether the patient has received any blood products or medications in the past month; the patient received no transfusions. Review of the medication list shows ceftriaxone and metronidazole. There is no prior blood bank record because he was transferred from another facility. The platelet count is 210 × 10^9/L.
1. What is the most likely cause of this ABO discrepancy, and what confirmatory approaches would you use before reporting the type?
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The pattern-strong A, weak B (1+), and a weak but present anti-B (2+) in a septic postoperative patient-fits the acquired B phenomenon. Bacterial enzymes (classically Escherichia coli O86, Klebsiella, or Proteus) deacetylate the A antigen's N-acetylgalactosamine to galactosamine, creating a B-like epitope. Because the patient is genetically group A, the immune system does not make a strong anti-B, so reverse anti-B is weak. The negative DAT argues against immune hemolysis. Confirm before reporting: (1) test with acidified anti-B (pH 6.0) or a monoclonal anti-B clone that does not react with acquired B; (2) perform ABO genotyping or test the serum against autologous cells-the patient's anti-B reacts with reagent B cells but not with his own cells. A weak B subgroup (e.g., B3) is less likely because the patient is septic, has colon cancer, and has a weak anti-B rather than the expected strong anti-B.
2. How should the blood bank report the ABO type and select RBC and plasma components? Would you give group A plasma or AB plasma?
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Report the patient as group A, Rh-positive, and note the acquired B in the comment. For RBCs, give group A units; they are ABO-compatible because the patient has no anti-A. Do not give group B or AB RBCs, which would be unnecessary and could complicate future typing. For plasma, AB plasma is preferred because group A plasma contains anti-B that can bind the acquired B antigen on the patient's RBCs and cause a positive crossmatch or mild hemolysis. If AB plasma is unavailable, group A plasma is acceptable when the anti-B titer is low and the patient is not massively transfused, but document the decision. The same logic applies to platelets: group A or AB platelets are acceptable, but AB is preferred if plasma-rich components are used. Emergency uncrossmatched release, if needed, should be group O RBCs and AB plasma.
3. If surgery cannot wait for confirmatory testing, what should be released, and what risks drive that decision?
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If surgery cannot wait, do not delay for confirmatory testing. Release group A RBCs if the antibody screen is negative and the immediate-spin crossmatch is compatible; if there is no time for crossmatch, release group O RBCs. For plasma, release AB plasma. Notify the surgeon and anesthesia that the patient has an acquired B phenotype and that group A RBCs and AB plasma are the intended products. Continue the confirmatory workup postoperatively. The main risk of delay is not ABO incompatibility but progression of sepsis and anemia; the main risk of using the wrong plasma is anti-B-mediated hemolysis of the patient's acquired B RBCs. Group O RBCs are a safe stopgap because they lack A and B antigens.