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Objective Proactive therapeutic drug monitoring (pTDM) intends to optimise tumour necrosis factor (TNF) inhibitor treatment through regular assessment of drug and antidrug antibody levels, enabling individualised dosing. However, health economic evidence for this strategy is limited. This study aims to evaluate the health benefits and costs of pTDM with infliximab compared with standard therapy.Methods Based on the 52-week randomised, controlled, open-label, multicentre Norwegian Drug Monitoring trial (NOR-DRUM) B trial, we estimated the cost-effectiveness of pTDM compared with standard infliximab maintenance therapy in patients with six different immune-mediated inflammatory diseases. For each patient (n=454), we estimated the 12-month healthcare costs, including pharmaceuticals, biochemical tests and other types of resource use. Reported in 2024 Euros, unit costs were based on market prices, diagnosis-related group cost weights and reimbursement schedules. We evaluated health outcomes with quality-adjusted life-years (QALYs), using the EuroQol five-dimension, three-level questionnaire (EQ-5D-3L) and Short Form six-dimension health utility measure (SF-6D). Indicator of cost-effectiveness was the incremental cost-effectiveness ratio, expressed as Euros/QALY, compared with an assumed cost-effectiveness threshold value of €23 755. We assessed sampling uncertainty using the non-parametric bootstrap.Results The mean 1-year cost per patient in the pTDM group was lower than in the standard therapy group, with a difference of €592 (95% CI −1304 to 107), while the QALYs based on EQ-5D-3L were 0.0018 higher (95% CI −0.0126 to 0.0165). Based on the bootstrap results, pTDM has a probability of 95% of being cost-effective. In explorative subgroup analyses, pTDM appeared cost-effective for some, but not all diagnoses.Conclusion pTDM is very likely a cost-effective treatment strategy for patients with immune-mediated inflammatory diseases on infliximab maintenance therapy.Trial registration number NCT03074656.