This is a comparative pragmatic trial in patients with UC and CD on maintenance treatment with IFX. All IBD patients from cohort A with maintenance IFX treatment at a referral IBD clinic are prospectively included between June and August 2018. An ultra-proactive IFX TDM algorithm is applied as follows. All patients have an ELISA TL measurement at baseline, of which the result determined the follow-up pathway: (A) TL between 3-7μg/mL: continuation at same dose and interval; (B) TL >7μg/mL: interval prolongation allowed; (C) TL <3μg/mL: interval shortening with minimum 2 weeks, with the next IFX TL measured using a POCT. (i) If the POCT showed an IFX TL <3μg/mL, dose was optimized ad hoc using a linear dosing formula (Dosen = (TL target * Dose n-1) / TL measured), followed by a new POCT test at next visit with the same interval. (ii) If the POCT showed an IFX TL ≥3µg/mL, no additional dose was given and routine TL testing with ELISA was retaken at next visit. At every visit this algorithm was reapplied to all patients.||The patients from cohort A will be compared with the patients from cohort B. The patients in cohort B receive maintenance IFX treatment at a referral IBD clinic during the same period. Dose adjustment are done based on standard of care reactive TDM of IFX and clinical symptoms. Data will be collected retrospectively to avoid treatment optimization bias.