Case report: Anti-p1 and AB blood group-A faux Para- Bombay

ABSTRACT

Background: Anti-P1 is a common antibody found in the sera of P2 donors, affecting one-quarter to two-thirds of those tested. Anti-P1 is an IgM isotype antibody that is frequently found as a weak cold agglutinin. Anti-P1 antibodies that are reactive at 37 Celsius or cause in vitro hemolysis are rare. With the exception of the rare Bombay phenotype, all red cells express the H antigen. The amount of H antigen on red cells is determined by an individual's ABO type since H antigen is the precursor to both A and B antigens. The expression of the H antigen is highest in group O and lowest in group A1B (O>A2 > B > A2B > A1 > A1B). We report a case of blood discrepancy mimicking Para-Bombay due to anti-P1 and weak H antigen expression in a 46-year-old Sarawak Malay blood donor during routine blood donor regrouping with an automated immunohematology analyser. She has history of COVID19 infection in September 2021 and she completed her 1st, 2nd and booster mRNA vaccine in November 2021. Her last pregnancy was 13 years ago, and she has no history of blood transfusions. Aims: To resolve blood group discrepancies detected when using an automated immunohematology analyser. To understand the possibility of interference from natural occurring cold-reacting red cell alloantibodies during indirect antiglobulin test blood grouping. To understand the possibility of false negative in forward grouping with anti-H antisera in donors with A1B blood group. Methods: Blood donor was typed for ABO and Rh by an automated immunohematology analyser with microplates. Serological methods for antibody detection and specification were done manually with column agglutination method (gel-card) and tube method. Results: Forward grouping of the donor's first sample with an automated analyser was strongly positive for Anti-A (4+), Anti-B (4+), Anti-AB (4+) and Anti-D (4+), while reverse grouping was also strongly positive for A1-cell (3+), B-cell (3+) and O-cell (4+). Manual serological methods with gel-card and tube method yielded similar results. Anti-H showed no reaction. The first sample was negative for Direct Coomb's test (DCT). The donor's second (repeat) sample using the manual serological method yielded similar results;however, reverse grouping repeated at 37 Celcius resulted in the cessation of reactions on known cells. Anti-H showed a 1+ reaction. Antibody screening was positive and proceeded to 11 panel antibody identification with Anti-P1 identified. DCT was negative in the second sample. (P1-) and Le(a-b+) are her phenotypes. Summary/Conclusions: Anti-P1 is commonly reported as cold reacting alloantibody in patients. In this case, a combination of strong reacting anti-P1 at room temperature and commonly low H antigen volume in A1B red cells lead to a false initial suggestion of Para- Bombay phenotype. Blood grouping discrepancies detected with automation should always be repeated manually.