An 83-year-old bedridden woman with a urinary drainage catheter for more than 3 years, hospitalised for advanced corticobasal degeneration, presented with a purple discolouration of the urine and the drainage bag (fig 1). Interestingly, the urine in the catheter from within the patient had a normal yellowish colour.

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Figure 1

Photograph shows a purple discolouration of the urine and the drainage bag.

This is a typical presentation of purple urine bag syndrome (PUBS). The discolouration is due to the presence of indigo and indirubin pigments, which interact with the plastic of the catheter urine bag.1 These pigments develop from the transformation indoxyl sulfate (a metabolite of tryptophan) because of the presence of urinary bacteria possessing indoxyl phosphatase/sulfatase activity.2 Thus, the syndrome is often associated with a urinary tract infection caused by Providencia bacteria, Escherichia coli, Proteus mirabilis or Klebsiella pneumonia.1 Other risk factors include constipation, alkaline urine and the use of plastic urinary catheters and bags.1 Women are at greater risk than men for this condition.1

PUBS can be distressing for patients, family members and healthcare workers who are unfamiliar with this phenomenon,3 although it is considered to be harmless and usually disappears after antibiotic treatment.2 Doctors should have knowledge of this rare phenomenon.