Do you remember what causes purple urine?

In medical school, we like to learn about zebras instead of horses. What I mean is that if you hear hoof beats, are you more likely to think about horses or zebras?  We simply don't see rare diseases (zebras) because we tend to see more common diseases (horses). What's common is common. What's common is also more likely to present in an uncommon fashion.

So, what causes purple urine?

Here's the explanation from the NEJM (New England Journal of Medicine):

Purple discoloration can occur in alkaline urine as a result of the degradation of indoxyl sulfate (indican), a metabolite of dietary tryptophan, into indigo (which is blue) and indirubin (which is red) by bacteria such as Providencia stuartii, Klebsiella pneumoniae, P. aeruginosa, Escherichia coli, and enterococcus species.

When's the last time you saw any purple urine? To read more about purple urine on the NEJM, click here