Utility of tolerance assays for real-time management of infliximab infusion reactions.

Failure of infliximab (IFX) therapy is often associated with pharmacokinetic challenges, frequently linked to the development of anti-infliximab antibodies (ATI). ATI formation is a key factor contributing to therapeutic failure and significant safety concerns, as these antibodies can trigger severe infusion-related reactions that may require treatment discontinuation. In Spain, most available ATI detection assays are sensitivity-based, identifying antibodies only in the absence of circulating IFX. In contrast, tolerance assays can detect ATI even in the presence of IFX, offering distinct advantages in managing immunogenicity. We report the case of a 29-year-old female with corticosteroid-refractory ulcerative colitis undergoing IFX treatment. The patient experienced an infusion reaction during her sixth dose. Blood tests revealed IFX trough levels of 10.6 µg/mL, free ATI <0.2 UA/mL, and total ATI >250 UA/mL. This case highlights the utility of tolerance assays in promptly assessing ATI during infusion reactions, enabling real-time therapeutic adjustments. While measuring free ATI is generally more informative in clinical practice, total antibody levels may provide valuable complementary insights, particularly in cases of suspected immunogenicity during infusion reactions.