Summary Background : Poor compliance with 5‐aminosalicylic acid therapy has been reported amongst patients with inflammatory bowel disease. Currently, there is no easy method to monitor 5‐aminosalicylic acid; however, the chemical similarity between 5‐aminosalicylic acid and salicylate might provide a solution. Aim : To determine the feasibility of using salicylate levels to monitor compliance with 5‐aminosalicylic acid medication. Methods : Thirty‐six patients with inflammatory bowel disease, taking maintenance 5‐aminosalicylic acid, provided either a paired serum and urine sample or an intestinal biopsy. Samples were split into two: half were sent to the hospital biochemistry department for salicylate measurement, and half were analysed for 5‐aminosalicylic acid and its metabolite, N‐acetyl‐5‐aminosalicylic acid, using high performance liquid chromatography. Correlation between the results was calculated. Results : Serum and urine were available for 25 patients. Serum salicylate was undetectable, but urinary salicylate ranged from 31 to 3254 µg/mL. The correlations between urinary salicylate and 5‐aminosalicylic acid and N‐acetyl‐5‐aminosalicylic acid were 0.96 (95% confidence interval, 0.91–0.98) and 0.9 (95% confidence interval, 0.77–0.96), respectively. Sixteen biopsies were available from 13 patients. The 5‐aminosalicylic acid and N‐acetyl‐5‐aminosalicylic acid concentrations were 0.2–657 ng/mg and 1.6–1598 ng/mg, respectively; there was no correlation with bowel salicylate. Conclusions : The close correlation between 5‐aminosalicylic acid and salicylate levels offers a simple method to assess compliance with 5‐aminosalicylic acid therapy.

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