Abstract: Objective To examine the serological features of a rare case of ABO blood type mosaicism and investigate the appropriate gene sequencing methodologies. Methods Serological tests, Sanger sequencing, and PacBio gene sequencing were used to identify the ABO blood type in a hospitalized patient exhibiting discrepancies in forward and reverse typing. Results Micro-column gel card retesting for forward typing showed double cell populations (DCP) with anti-A reagent and a negative result with anti-B reagent, while reverse typing confirmed type "A". Rh blood group antigens D, C, c, E, and e were not detected as DCP. Using the tube method, the patient's red blood cells (RBCs) showed "2+^w" agglutination with anti-H reagent, whereas no agglutination was observed with anti-A reagent, but a "4+" reaction was seen with anti-H, indicative of a notable difference compared to the gel method. Sanger sequencing yielded inconsistent results acroos the two tests, revealing two different haplotypes: ABO*O.01.01/ABO*O.01.01 and ABO*A1.02/ABO*O.01.01. PacBio sequencing identified a primary haplotype of ABO*O.01.01/ABO*O.01.01 with approximately 400 reads, along with a haplotype of ABO*A1.02, amounting to about 40 reads. Conclusion Serological tests may fail to identity ABO blood type chimerism, and Sanger sequencing and genotyping may not detect mosaicism. However, PacBio third-generation sequencing offers a more accurate method for identifying ABO blood type and quantifying the proportions of haplotypes in chimeric or mosaic blood types.

Key words: ABO blood type, Chimera, Mosaicism, Sanger sequencing, PacBio sequencing