Primary and secondary response loss , often mediated by the development of anti-drug antibodies (ADAs), limit treatment efficacy. Therapeutic drug monitoring (TDM) is important in optimizing infliximab therapy by correlating drug level with clinical outcomes. This study aimed to find the relationship between infliximab level, ADA presence, and clinical activity in Iraqi Crohn's disease (CD) patients. This cross-sectional study included adult Crohn's disease patients receiving maintenance infliximab (IFX) treatment. Data on demographics and clinical characteristics were collected. Infliximab level and ADA were measured using ELISA. Forty-three patients were included. Patients in remission had significantly higher infliximab levels and lower ADA levels compared to those with active disease. The CD Activity Index (CDAI) correlated negatively with infliximab levels and positively with ADA levels . Infliximab levels correlated negatively with treatment duration and ESR , while positively correlating with the infliximab dose. ADA levels correlated positively with treatment duration. This study, which investigated IFX and ADA levels in CD patients during maintenance therapy, observed a remission rate of approximately 50% in patients. The mean IFX plasma level was 3.28±5.19, with significantly higher levels in remission group compared to active disease (4.91±6.65 vs. 1.73±2.56). This study found no significant difference in response between patients using these medications and those who weren't. Notably, active disease patients showed high ADA and low IFX levels, with a negative correlation between CDAI and IFX levels and a positive correlation between CDAI and ADA levels. These findings, consistent with other research, underscore the critical need for therapeutic drug monitoring (TDM) to personalize IFX dosing, aiming to improve patient response rates, reduce overall treatment costs, and minimize adverse events from concomitant medications, particularly in regions like Iraq where TDM implementation is currently limited. This study concluded that infliximab level was significantly linked to Crohn's disease clinical activity. Patients in remission had higher infliximab level and lower anti-drug antibody level, indicating the importance of TDM in personalize IFX treatment for better results.
