Routine measurement of infliximab levels and anti-drug antibodies before the 4th infliximab dose is recommended by ESPGHAN in paediatric IBD. This proactive approach is intended to prompt early optimisation of dosing during the maintenance phase. In our Children’s Hospital we routinely measure infliximab trough levels and anti-drug antibodies prior to the 4th dose. We aimed to assess if this practice leads to changes in treatment post-induction.

Patients aged 0−17 years commencing infliximab between January 2021 and May 2022 who had trough infliximab levels and anti-drug antibodies measured at their 4th dose were included. Infliximab level ≥5 mg/L was considered therapeutic and the threshold for antibody positivity was >10AU/L. Outcome data were collected from our hospital database including demographics, biochemistry, and clinical course to assess for a change of infliximab dose/frequency, switch of biologic or introduction/increase of thiopurine.

Of the 44 patients that met inclusion criteria, infliximab levels were sub-therapeutic in 27 (61%) and antibodies were elevated in 17 (39%). Treatment was changed for 21 patients (48%) following therapeutic drug monitoring (TDM) and both low infliximab levels and elevated antibodies independently predicted subsequent treatment escalation (p=0.012 and p=0.023 respectively). The most common change was to increase infliximab dose from 5 mg/kg to 10 mg/kg (14/21). Of the 23 patients whose treatment was not changed, 14 (61%) had both therapeutic infliximab levels and negative antibodies. Serum albumin correlated positively with infliximab level (p=0.035) and negatively with CRP (p=0.0001) but neither CRP, albumin, age, underlying diagnosis or thiopurine use predicted subsequent treatment change.

Routine measurement of infliximab levels and anti-drug antibodies before the 4th dose was associated with treatment change in a substantial proportion of patients, the majority of whom increased infliximab dose. These data reinforce the importance of optimising infliximab dose based on TDM to achieve ‘adequate’ levels, although longer follow-up is needed to understand how these early adjustments affect clinical outcome.