AIMS: We aimed to evaluate the association between 6-thioguanine nucleotide (6-TGN) and anti-TNFα levels and the optimal 6-TGN threshold level associated with higher anti-TNFα levels in combination therapy.

METHODS: We performed a retrospective cross-sectional multicentre study of patients with IBD on combination anti-TNFα and thiopurine maintenance therapy between January 2015 and August 2021. Primary outcomes were infliximab and adalimumab levels. Secondary outcomes were antibodies to infliximab (ATI) or adalimumab (ATA). Univariable and multivariable linear regression were performed to identify variables associated with anti-TNFα levels. Receiver operator characteristic curves were used to define the optimal 6-TGN cut-off levels associated with therapeutic anti-TNFα levels.

RESULTS: The study included 743 paired 6-TGN and anti-TNFα levels (640 infliximab and 103 adalimumab). 6-TGN levels were associated with infliximab levels, but not adalimumab levels, on univariable and multivariable regression. The optimal 6-TGN cut-off associated with therapeutic infliximab levels (≥5 mcg/mL) was 261 pmol/8 × 108 red blood cell (RBC) (area under the curve (AUC) = 0.57) for standard infliximab dosing and 227.5 pmol/8 × 108 RBC (AUC = 0.58) for escalated dosing. For therapeutic adalimumab levels (≥7.5 mcg/mL), the 6-TGN cut-off was 218.5 pmol/8 × 108 RBC (AUC = 0.59) for standard adalimumab dosing and 237.5 pmol/8 × 108 RBC (AUC = 0.63) for escalated dosing.