*Purpose: ABO antibody (ABO-Ab) quantification is essential for risk assessment for ABO-incompatible (ABOi) transplantation, but the hemagglutination assay (HA) for ABO-Ab titres is known to be plagued by poor reproducibility and variable sensitivity. Our aim was to quantify IgG and IgM ABO-Ab as Luminex mean fluorescence intensities (MFI) values and compare to HA titres.

*Methods: Healthy adult samples from all ABO groups (n=87; 60% female) were tested by “standard” HA (no AHG). Serially diluted sera (50uL) were incubated with 25uL of 1% A1 and B reagent red cells at room temperature; HA titre was reported as the last dilution showing visual agglutination. The same samples were tested against in-house developed Luminex ABO single antigen beads (ABO-A and -B subtype I-VI glycans were coupled to individual beads). ABO-Abs bound to the Luminex beads were detected with either anti-human IgG or IgM secondary antibodies.

*Results: Although a pattern of increasing median IgM MFI vs HA titres was evident (Fig 1a,1c), each titre reflected a wide range of IgG and IgM MFI (Fig 1b, 1d). No significant sex-based differences in titres or MFI were detected (not shown).

*Conclusions: These results suggest that HA titre alone is insufficient to accurately assess ABOi transplant risk. IgG antibodies can contribute to non-AHG HA titres and each ABO titre includes an unpredictable range of ABO-Abs. Tools such as this bead-based ABO assay will allow more accurate ABOi transplant risk assessment and inform the relative contribution of specific ABO-Ab isotypes.

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