Age as a Predictor of Serum Tumor Necrosis Factor Antagonist Drug and Antidrug Antibody Concentrations in Inflammatory Bowel Disease—A Nationwide Cohort Study

<b>Background/Objectives</b>: Tumor necrosis factor antagonists (anti-TNFs) have been shown to be an effective treatment for inflammatory bowel disease (IBD). Several factors are associated with anti-TNF treatment failure. This study aims to explore the impact of age on serum concentrations of anti-TNF drugs and antidrug antibodies (ADAbs). <b>Methods</b>: We retrospectively reviewed patients' charts from July 2018 until September 2024 across seven medical centers. Patients with an established diagnosis of IBD receiving infliximab or adalimumab were included. The primary outcome of this study was the effect of age on the anti-TNFs serum drug concentration and ADAb levels. Linear regression was performed to explore the relationship between age and serum anti-TNF drug and ADAb levels. <b>Results</b>: 1093 patients were included in our cohort. In patients receiving infliximab, there was a significant association between older age and increasing ADAbs levels (<i>p</i> = 0.036), whereas in patients treated with adalimumab, there was no significant relationship between older age and ADAb levels (<i>p</i> = 0.771). There was no significant relationship between age and adalimumab serum concentration (<i>p</i> = 0.54). When stratified by age, patients taking infliximab who were >30 years of age developed more ADAbs compared to those aged ≤30 (<i>p</i> = 0.003). <b>Conclusions</b>: Patients older than 30 years of age receiving infliximab monotherapy have higher ADAbs and lower serum drug concentrations than younger patients. There was no statistically significant difference in ADAbs and serum drug concentrations among patients receiving infliximab combination therapy or adalimumab monotherapy.