Background: Primary non-response and secondary loss of response (LOR) are significant problems of biological therapy for inflammatory bowel disease (IBD). Therapeutic drug monitoring (TDM) in IBD patients receiving these drugs can improve outcomes. Aim: To measure serum infliximab levels and anti-infliximab antibodies (ATI) in patients with IBD post-induction phase and during maintenance therapy assessing the clinical course of IBD. Patients and methods: Prospective study of IBD patients receiving infliximab between July 2016-May 2017. Group-A included patients who received induction therapy while Group-B included patients who were in maintenance therapy. TDM was performed in serum samples collected at weeks-14 and 30 in Group-A and before the infliximab maintenance dose in Group-B. Clinical scores, fecal calprotectin and endoscopic score were also evaluated. Results: Of 14 patients in Group-A, 57% achieved endoscopic response. Median serum infliximab concentrations at week-14 and 30 were 2.65 AU/mL (0.23-32.58) and 2.3 AU/mL (0.3-16.8), respectively. Patients with mucosal healing had non-significantly higher median infliximab concentrations at week-14, as compared to week 30 (median 3.2 vs 2.2 AU/ml, respectively, p 0.6). ATI >10 ug/mL were found in one and seven patients at week-14 and 30, respectively. At 52 weeks of follow-up, four patients (31%) had LOR. Group-B included 36 patients, 33% had LOR. Median serum concentrations of infliximab were 1.4 AU/mL (0.27-7.03). No significant differences in serum infliximab concentration were observed between patients in remission and those with inflammatory activity. Seventeen patients had ATI >10 ug/mL. Conclusions: Clinical algorithms using TDM might help to optimize the pharmacological therapy of IBD.Background: Primary non-response and secondary loss of response (LOR) are significant problems of biological therapy for inflammatory bowel disease (IBD). Therapeutic drug monitoring (TDM) in IBD patients receiving these drugs can improve outcomes. Aim: To measure serum infliximab levels and anti-infliximab antibodies (ATI) in patients with IBD post-induction phase and during maintenance therapy assessing the clinical course of IBD. Patients and methods: Prospective study of IBD patients receiving infliximab between July 2016-May 2017. Group-A included patients who received induction therapy while Group-B included patients who were in maintenance therapy. TDM was performed in serum samples collected at weeks-14 and 30 in Group-A and before the infliximab maintenance dose in Group-B. Clinical scores, fecal calprotectin and endoscopic score were also evaluated. Results: Of 14 patients in Group-A, 57% achieved endoscopic response. Median serum infliximab concentrations at week-14 and 30 were 2.65 AU/mL (0.23-32.58) and 2.3 AU/mL (0.3-16.8), respectively. Patients with mucosal healing had non-significantly higher median infliximab concentrations at week-14, as compared to week 30 (median 3.2 vs 2.2 AU/ml, respectively, p 0.6). ATI >10 ug/mL were found in one and seven patients at week-14 and 30, respectively. At 52 weeks of follow-up, four patients (31%) had LOR. Group-B included 36 patients, 33% had LOR. Median serum concentrations of infliximab were 1.4 AU/mL (0.27-7.03). No significant differences in serum infliximab concentration were observed between patients in remission and those with inflammatory activity. Seventeen patients had ATI >10 ug/mL. Conclusions: Clinical algorithms using TDM might help to optimize the pharmacological therapy of IBD.