What is the cause of purple urine in a patient with a Foley catheter and what are the appropriate next steps in management?

Purple Urine in a Foley Catheter: Purple Urine Bag Syndrome

Purple urine draining from a Foley catheter represents Purple Urine Bag Syndrome (PUBS), a benign but alarming manifestation of catheter-associated urinary tract infection that requires catheter replacement and appropriate antibiotic therapy based on urine culture results. 1, 2

Pathophysiology and Causative Organisms

PUBS occurs when bacteria possessing indoxyl sulfatase enzymes (primarily Escherichia coli, Proteus mirabilis, Klebsiella pneumoniae, Enterococcus faecalis, and Proteus vulgaris) colonize the urinary catheter and metabolize tryptophan-derived indoxyl sulfate into indirubin (red) and indigo (blue) pigments that mix to create the characteristic purple discoloration in the plastic catheter tubing and collection bag 2, 3. This biochemical process requires alkaline urine pH and bacterial enzymatic activity 2.

Immediate Management Steps

1. Replace the Catheter Immediately

• Remove and replace the current Foley catheter with a fresh catheter, as PUBS is often the earliest indicator of catheter malfunction, biofilm formation, or obstruction requiring immediate intervention 1, 4.
• Use an appropriately sized catheter (14-16 Fr for adults) to minimize urethral trauma while maintaining adequate drainage 5.
• Consider silver alloy-coated catheters if prolonged catheterization remains necessary, as these reduce infection risk 6, 5.

2. Obtain Urine Culture Before Antibiotics

• Collect urine culture from the freshly placed catheter before initiating antimicrobial therapy to identify the causative organism and guide targeted antibiotic selection 7, 5.
• Do not culture urine from the old catheter or collection bag, as this will reflect biofilm colonization rather than active infection 7.

3. Assess for Symptomatic UTI vs. Asymptomatic Bacteriuria

• Evaluate for systemic signs of infection including fever, dysuria, suprapubic pain, flank pain, altered mental status, or hemodynamic instability that would indicate symptomatic catheter-associated UTI requiring treatment 3, 4.
• In asymptomatic catheterized patients without neutropenia, critical illness, or immunosuppression, treatment of bacteriuria has never been shown to provide benefit and catheter replacement alone may suffice 7.
• However, PUBS typically indicates significant bacterial colonization and warrants treatment in most clinical scenarios, particularly in elderly, constipated, or immobilized patients 3.

4. Initiate Empiric Antibiotics for Symptomatic Patients

• Start empiric broad-spectrum antibiotics covering gram-negative organisms (E. coli, Proteus, Klebsiella) and enterococci while awaiting culture results 2, 4.
• Treat for 7-10 days depending on symptom resolution and culture sensitivities 7.
• Adjust antibiotics based on culture results and local antibiogram patterns 2.

Address Predisposing Risk Factors

Constipation Management

• Aggressively treat constipation with laxatives, as this is a major modifiable risk factor for PUBS and resolving constipation may prevent recurrence 5, 3.
• Constipation promotes bacterial overgrowth and alkaline urine pH that facilitates the biochemical conversion to purple pigments 3.

Catheter Duration Minimization

• Remove the Foley catheter within 24-48 hours if clinically feasible to eliminate the nidus for bacterial colonization and prevent recurrent infection 8, 6, 5.
• If the catheter cannot be removed, implement intermittent catheterization every 4-6 hours as an alternative to indwelling catheterization, which reduces infection risk 8, 6.

Alkaline Urine Correction

• Consider urinary acidification strategies if alkaline urine persists, as bacterial indoxyl sulfatase activity is enhanced in alkaline environments 2.

Clinical Significance and Prognosis

• PUBS itself is clinically benign and rarely causes sepsis, but it serves as a visual marker of significant catheter-associated bacteriuria that may progress to symptomatic UTI or urosepsis if left untreated 2, 3.
• The purple discoloration resolves spontaneously after catheter replacement and appropriate antibiotic therapy 1, 2.
• PUBS can occur with any indwelling urinary drainage system including nephrostomy tubes and suprapubic catheters, not just urethral Foley catheters 1, 9, 4.

Common Pitfalls to Avoid

• Do not dismiss purple urine as a harmless curiosity—it indicates catheter malfunction and significant bacterial colonization requiring intervention 1.
• Do not use prophylactic antibiotics routinely in catheterized patients, as this selects for resistant organisms without proven benefit 7, 5.
• Do not delay catheter replacement while waiting for culture results, as the biofilm-coated catheter perpetuates infection 1, 2.
• Do not forget to evaluate for catheter obstruction, as PUBS may be the first sign of drainage system malfunction 1.

High-Risk Patient Populations

PUBS occurs predominantly in:

• Elderly, institutionalized patients with long-term catheterization 2, 3
• Chronically constipated and immobilized individuals 3
• Patients with spinal cord injury, dementia, or chronic kidney disease 3, 4
• Female patients (more common than males) 3