A 58-year-old man with an indwelling urinary catheter began producing urine with a deep purple colour (Figure 1). The urine was alkaline (pH 7.8), and Proteus mirabilis urinary tract infection was diagnosed. This so-called purple urine bag syndrome resolved after treatment with ceftazidime. The phenomenon occurs over a 2-year period in about 8% of patients with indwelling urinary catheters.¹ The syndrome is often associated with a urinary tract infection caused by Providencia bacteria, Escherichia coli, P. mirabilis or Klebsiella pneumoniae. These bacteria produce an enzyme (indoxyl phosphatase/sulfatase) that converts the excreted indoxyl sulfate in the urine into red-and blue-coloured compounds (indirubin and indigo), which mix to form the purple colour.

Tryptophan normally exists in the intestine and is degraded into indoxyl sulfate. This in turn is absorbed and excreted in the urine. Constipation, which prolongs tryptophan transit time in the intestine, results in increased indoxyl sulfate levels in urine and is a risk factor for purple urine bag syndrome. Other risk factors include alkaline urine (which predisposes to the growth of contributory microbes) and the use of catheters made of polyvinyl chloride plastic.¹ Women are at greater risk than men for this condition.

Urine colour may change for many reasons. Purple urine can be associated with intestinal intussusception.² A green colour can occur with Pseudomonas urinary tract infections or if a patient has taken methylene blue, amitriptyline, indomethacin, doxorubicin or propofol. A deep orange colour can be associated with treatment with rifampin or warfarin.

Che-Kim Tan MD Department of Intensive Care Medicine Chi-Mei Medical Center Tainan, Taiwan Department of Internal Medicine Taipei Medical University Taipei, Taiwan Ya-Ping Wu RT Hsing-Ying Wu RT Chih-Cheng Lai MD Department of Internal Medicine Yi-Min Hospital Taipei, Taiwan