A 70-year-old woman had a urinary catheter placed during a hospitalization for aphasia and hemiplegia after an acute stroke. Ten days after catheterization, her urine appeared purple. Urinalysis showed leukocytes but no nitrites, and the pH was 8.
The purple discoloration is thought to occur as a result of a chemical reaction facilitated by certain bacteria in alkaline environments.
Dietary tryptophan is converted to indole within the gut, absorbed into the circulation, and metabolized by the liver to form indoxyl sulfate. When excreted in the urine, indoxyl sulfate can be broken down by bacterial enzymes to form indigo and indirubin, which are blue and red, respectively, creating the color purple when combined.