Purple urine bag syndrome: a rare and interesting phenomenon.

Discoloration of urine is not uncommonly encountered in clinical practice and may indicate a significant pathology. However, the majority of instances are benign and occur as the result of trauma to the urological system during procedures or ingestions of substances such as medication or food. Purple discoloration of a urinary catheter bag is rare and can be alarming to both patients and healthcare workers. This phenomenon is known as the purple urine bag syndrome. It is associated with urinary tract infections occurring in catheterized patients, generally elderly females with significant comorbidities and constipation. The urine is usually alkaline. Gram-negative bacteria that produce sulfatase and phosphatase are involved in the formation of pigment, indirubin and indigo. Tryptophan metabolism is involved in the pathogenesis. We present two cases of this rare and interesting phenomenon and discuss the underlying pathogenesis.