Rare chemical reaction turns French woman’s urine bright purple

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Doctors in France recently detailed an odd case involving a 70-year-old woman’s urine turning bright purple.

The woman, who was not identified in the case report published Thursday in the New England Journal of Medicine (NEJM), was hospitalized after suffering a stroke. About 10 days after she was fitted with a urinary catheter, her pee turned purple — what doctors with the Hôpital de Bicêtre in Le Kremlin-Bicêtre said was a result of what’s known as “purple urinary bag syndrome,” or PUBS.

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Though seeing your urine the color of an eggplant can be alarming, PUBS is actually considered a “benign phenomenon,” according to a 2017 report on the syndrome.

PUBS, which was first reported in 1978, according to the journal Annals of Long-Term Care, is “thought to occur as a result of a chemical reaction facilitated by certain bacteria in alkaline environment,” according to the case report. It’s more common in elderly adults who are catheterized for a long period of time and also have a urinary tract infection (UTI).

Other risk factors include "female gender, constipation and chronic renal failure,” per a 2018 BMJ report.

PUBS can occur after certain bacteria mixes with the chemical tryptophan, an amino acid commonly found in foods such as turkey and cheese, among others.

When tryptophan enters the gut, it’s converted into indole, a chemical compound. Indole then becomes indoxyl sulfate after it's metabolized by the liver.

When indoxyl sulfate and certain bacteria — specifically Klebsiella pneumoniae in the woman’s case, a urine culture showed — mix in the urinary bag after expelled from the body, a purple color can occur.

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“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,” the doctors explained in the case study, noting other bacteria such as Pseudomonas aeruginosa, Providencia stuartii, and Escherichia coli can also produce a similar effect.

As previously stated, PUBS can be a sign of a UTI. But doctors treating the French woman found “no clinical signs consistent with infection” and instead treated her with intravenous hydration. Four days later, her urine returned to a normal color.