65 yr old man presented to the office complaining that his urine is blue after realizing that he was not joking I asked if this was something new to which he replied that it began about 2 months ago. He added that it was painless and without any fever or chills and denied any new medication which excluded inflamatory and drugs as causes respectively.
Upon further inquiry he stated that he was diagnosed with bladder cancer a few years ago and that he has a diverting ileal conduit that is connected to a urine collection bag attached to his ankle.
The plot thickens
I then asked if the urine was blue in the tubing to which he replied, “No, it turns blue after getting in the bag”. This suggested that there was some kind of chemical reaction in the bag that turned yellow urine blue.
I asked if he had any food fads recently to which he replied that he really took a liking to a particular mint flavored candy. A quick search revealed that mint contains large quantities of tryptophan.
The mystery solved
Tryptophan gets converted to Indole by intestinal bacteria which in turn is converted to indoxyl sulfate through hepatic conjugation. This is water soluble and excreted in the urine and is colorless. Indoxyl sulfate is then broken down by bacteria in the urine collection bag to indigo which is blue. Hence the blue urine.
Every time he was put on antibiotics he noted that the blue color temporarily resolved as a result of the antibiotics accumulating in the urine.