Purple Urine Bag Syndrome (PUBS Syndrome): Tryptophan
(l-tryptophan) is a rarely used antidepressant drug. It decomposes
in the gut to form indoxyl sulfate, which is absorbed and secreted in the
urine (indican). Urinary organisms, such as Proteus and Klebsiella, have indoxyl sulfatase
activity to metabolize indican to indigo, particularly in alkaline urine. In
the other suggested pathway, once indoxyl sulfate comes in contact with air,
it is oxidized to insoluble indigo, which causes the deep purple
discoloration of the urine, the catheter, and the urine bag. Enteritis,
pancreatic insufficiency, ileus, peritonitis (increased production), renal
insufficiency (diminished excretion), bile acids, and steroidal conjugates
also can cause a purple discoloration of the urine. Other reports suggest
the color results from an interaction between indigo crystals in the urine
and indirubin dissolved in the plastic bag for the urine. Most often, except
for the scary color, this finding has no clinical relevance.