Therapeutic Drug Monitoring in Special Circumstances in Inflammatory Bowel Disease

Inflammatory bowel disease, encompassing ulcerative colitis and Crohn's disease, is characterised by chronic immune-mediated inflammation and variable treatment response. Loss of drug efficacy due to underexposure, pharmacokinetic variability, and immunogenicity remains a key challenge. Therapeutic drug monitoring, using drug levels and anti-drug antibody measurements, is an important strategy for optimising the treatment of inflammatory bowel disease. It helps ensure adequate dosing and can distinguish between pharmacokinetic and mechanistic drug failure. Most evidence pertains to infliximab and adalimumab. Multiple factors influence drug pharmacokinetics, affecting both target drug levels and the doses required to achieve them. These include inflammatory burden, bodyweight, age, disease phenotype, and route of administration, all of which are important considerations for individualising treatment in inflammatory bowel disease. This narrative review explores how special clinical situations-acute severe ulcerative colitis, perianal fistulising Crohn's disease, hypoalbuminaemia, extremes of body composition, pregnancy, paediatrics, and advanced age-alter drug pharmacokinetics and influence the utility and interpretation of therapeutic drug monitoring in inflammatory bowel disease.