The effect of hypochlorous acid on urine color among pediatric ulcerative colitis patients treated with mesalamine

BACKGROUND: The use of mesalamine for the treatment of ulcerative colitis (UC) is common, and may rarely cause nephrotoxicity. One of the clinical signs of nephropathy is hematuria or dark urine. We aimed to evaluate the effect of hypochlorous acid (HA) found in household bleach on the color of urine among pediatric UC subjects treated with mesalamine. METHODS: This was a prospective case-control study. We obtained fresh urine samples from UC subjects in remission and with intact kidney function. A urinalysis was used to rule out hematuria or signs of infection. We noted the color of the urine before and after adding 1mL of HA to each sample. RESULTS: Our study included 17 UC subjects, of them 14 (82%) were females with a median age of 17 years (IQR 14.5-17.5 years). Nine (53%) of subjects were on mesalamine therapy with a median daily dose of 64 mg/kg, while eight (47%) received other therapies (three infliximab, three adalimumab, one vedolizumab, one ustekinumab). Initial urine color of all samples was between transparent and dark-yellow. All subjects taking mesalamine had an immediate and notable change of urine color to brown compared to none of the subjects treated with other drugs (P=0.0001). This phenomenon was unaffected by the initial urine color, daily dose taken or time from last mesalamine dose. CONCLUSIONS: Urine of mesalamine users turns brown when exposed to HA, which may cause unnecessary alarm among such patients. Likewise, awareness of this benign phenomenon may help avoid needless alarm and testing.