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The aim of this study was to assess whether sustained 6-thioguaninenucleotide (6-TGN) levels were associated with improved long-term outcomes in patients with inflammatory bowel diseases (IBD).Cross-sectional data have shown that thiopurine metabolites are correlated with clinical efficacy in patient receiving thiopurines for IBD but the role for serial measurements through treatment course is unclear.We conducted a retrospective cohort study including patients with IBD on thiopurine monotherapy and had serial 6-TGN levels measured. Predictive variables included demographics, disease phenotype, 6-TGN levels (nadir, median, and peak levels). The primary outcome was the development of a disease relapse. The secondary outcome was the need for IBD surgery.Two hundred eighteen 6-TGN samples from 87 patients were analyzed. Nadir and median 6-TGN levels were significantly higher in patients who did not relapse [185 and 233 pmol per 8×108 red blood cells (RBCs)] as compared with levels in patients who did relapse (150 and 167 pmol per 8×108 RBCs, P=0.025) but there was no significant difference in peak 6-TGN level. When adjusted for confounding factors, a nadir 6-TGN level ≥161 and a median 6-TGN level ≥264 were associated with a significant decrease in the rate of disease exacerbation (hazard ratio: 0.5; 95% confidence interval, 0.26-0.87; P=0.016 and hazard ratio: 0.4; 95% confidence interval, 0.2-0.82; P=0.14).Serial thiopurine metabolite level assessments and dose adjustment aiming to maintain higher 6-TGN levels may be helpful to improve long-term outcomes in patients with IBD.