A 66-year-old man arrived at an emergency department with sudden penile pain after catching his Foley catheter in the shower. He was not there about the color of anything.
On examination, clinicians found something that demanded an explanation. He had two separate urinary drainage systems in place. The Foley catheter running from his bladder was draining ordinary yellow urine. The percutaneous nephrostomy tube draining his left kidney was filling its bag with purple.
Same patient. Same day. Same urine supply, split between two exits, and only one had changed color.
The case, published in April by Sarah Griffin and colleagues in the Journal of Brown Hospital Medicine, documents purple urine bag syndrome in a setting where it is rarely described. The corresponding author is based at Louisiana State University Health Sciences Center in New Orleans.
The Chemistry That Turns a Bag Violet
Purple urine bag syndrome was first reported in 1978. Its true incidence remains unknown, and a review of the phenomenon traces the earliest recorded observation to 1812, when physicians caring for King George III noted a bluish tint in his urine.
The pathway starts at the dinner table. Tryptophan, an essential amino acid found in ordinary food, is broken down by gut bacteria into indole. The liver converts indole into indoxyl sulfate, which the kidneys excrete into urine.
If the right bacteria are present in the urinary tract, bacterial sulfatase and phosphatase enzymes split indoxyl sulfate into two pigments. Indigo is blue. Indirubin is red. Mixed in varying proportions against the plastic of catheter tubing and collection bags, they produce shades running from purple through blue to reddish. Polyvinyl chloride has been specifically implicated, which is why the container often looks more dramatically stained than the urine itself.
Alkaline urine is usually described as a requirement, though the review notes at least one published case with acidic urine, which casts some doubt on how absolute that rule is.
Constipation feeds the same pathway. Slower transit gives gut bacteria more time to convert tryptophan into indole, raising the indoxyl sulfate that eventually reaches the urine. Reduced kidney clearance of indoxyl sulfate has a comparable effect, which is why chronic kidney disease appears repeatedly among the reported risk factors. King George III, as it happens, also had chronic constipation.
Why Two Bags in One Body Told Different Stories
The patient's history explains how he ended up with two drainage routes. He had stage IIIB rectal adenocarcinoma treated with neoadjuvant chemotherapy and radiation, followed by low anterior resection surgery. That surgery produced complications including bladder and ureteral injury, requiring repair, a left percutaneous nephrostomy tube, and an indwelling Foley catheter. Both had been in place about a month.
His temperature was 97.4 degrees Fahrenheit, but his heart rate was 120, and an EKG confirmed sinus tachycardia. His white blood cell count was normal at 9,000 cells per microliter, and his kidney function was normal with a creatinine of 0.84.
Imaging found real structural problems. A CT urogram showed a 3.4-centimeter rim-enhancing fluid collection containing air in the retroperitoneum, and a CT cystogram showed contrast leaking from the posterior bladder dome into a presacral collection. Urology advised keeping the Foley in place given that fistula.
Urine culture grew more than 100,000 colony-forming units of three pan-sensitive organisms: Klebsiella pneumoniae, Proteus mirabilis, and Enterococcus gallinarum. The first two appear repeatedly in the purple urine literature.
The split between the two bags points to the answer. The color depends on bacteria colonizing the drainage system, not on something circulating in the blood. One tube had accumulated the necessary bacterial population and one had not. In a series of 46 patients at one tertiary center, nephrostomy tubes accounted for about a quarter of cases, with urethral catheters making up most of the rest.
A case-control study in Japanese geriatric wards found no correlation between any specific bacterial strain and the syndrome. What it did find was that high bacterial counts, particularly above 100,000 colony-forming units per milliliter, were present in most samples from affected patients, alongside higher rates in women and in those with alkaline urine.
The Decision Not to Treat
The most instructive part of this case is what the team chose not to do.
The patient was admitted and started on broad-spectrum antibiotics, vancomycin and piperacillin-tazobactam. Infectious diseases specialists were consulted. They recommended against treating for urinary tract infection, because he had no symptoms of one and his penile pain had resolved once the Foley catheter was replaced.
Antibiotics were discontinued on day two. The nephrostomy tube continued to drain purple urine, attributed to bacterial colonization, and he went home with outpatient follow-up planned.
The authors were explicit that the finding alone is not an indication for antibiotics. It signals bacteria in the urine, which may represent colonization rather than infection, and patients should be screened for actual symptoms before treatment. Discoloration typically clears once the catheter and bag are exchanged or the bacteria are treated.
The same finding has been reported in patients with prolonged nephrostomy tubes in palliative care and, occasionally, in people with no urinary catheter at all.
What Patients and Families Should Take from This
For anyone caring for a family member with a long-term catheter, the practical message is reassurance paired with a phone call. The color is alarming, and it is generally benign, but it should be reported to the clinical team rather than ignored, since it does indicate substantial bacterial growth.
The authors' concern was that the striking appearance drives unnecessary testing and treatment. Their case shows how to hold that line without ignoring everything else: the purple bag was left alone, while the bladder leak it distracted from was not.
This is a single case report. It does not change any guideline. Decisions about antibiotics belong with the treating clinicians, who can assess whether symptoms of infection are actually present.
Key Questions Answered
What makes the urine turn purple?
Bacteria in the urinary tract convert indoxyl sulfate, a breakdown product of dietary tryptophan, into two pigments. Indigo is blue, and indirubin is red, and together they produce purple, usually in alkaline urine.
Why did only one of this patient's two bags change color?
The pigments are produced by bacteria colonizing the drainage system itself. His nephrostomy tube had the bacterial population needed to generate them, and his Foley catheter did not.
Is purple urine bag syndrome dangerous?
It is generally considered benign. It indicates a high bacterial count in the urine, which can represent colonization rather than active infection.
Was this patient given antibiotics?
He was started on them and then taken off after two days. Infectious disease specialists advised against treating for urinary tract infection because he had no symptoms of one.
Who is most likely to develop it?
Reported risk factors include advanced age, female sex, chronic debilitation, dementia, constipation, alkaline urine, chronic kidney disease, and prolonged catheterization.
What should a caregiver do if they see it?
Contact the clinical team. The discoloration is usually harmless, but it does signal bacterial growth and typically clears after the catheter and bag are changed.
