An elderly woman presented with obstructive uropathy requiring a nephrostomy. After ureteric stenting the urinary catheter bag turned purple but the nephrostomy bag remained clear. Reports suggest that the discoloration is due to a mixture of indirubin dissolved in the plastic and indigo on its surface. Organisms in the urine that possess indoxyl sulphatase activity, such as Providencia and Klebsiella, metabolise urinary indoxyl sulphate to indigo leading to purple urine bag syndrome. This is often seen with elderly patients as they have high levels of urinary indoxyl sulphate. No organism was identified in this patient, but she had cystitis without evidence of ascending infection.

Stephen G Riley, specialist registrar, Prem Thurairajah, senior house officer, Bryonie F Read, senior …