Something Bizarre in a Urine Collection Bag

HISTORY

A staff nurse calls the physician to see "something really bizarre—we have no idea what this is": the plastic of a urine collection bag of an 81-year-old woman has turned purple, although the urine within is more orange-brown and distinctly less odd looking. Patient has had an indwelling urethral catheter for 7 months, and this discoloration had not been noted before. Current history includes marked constipation, atrial fibrillation, and symptomatic osteoarthrosis of the hips and knees.

PHYSICAL EXAMINATION

Patient is lucid but frail; says she has felt mild suprapubic tenderness on and off for the past month. Temperature is 38°C (100.4°F).

What's Your Diagnosis?

This syndrome results from infection or sympomatic bacteriuria, but its very name sounds like a jest. It affects a considerable number of persons, for many of whom discoloration of the plastic is more intense than that of the urine. It is the subject of numerous reports, of which a small subset are cited here,^1-13 yet it remains unfamiliar to most clinicians.

BEAUTIFULLY WORKED-OUT CHEMISTRY

Although descriptions of the staining of bags and urine go back to 1978,^1-3 the effect of indigo is barely alluded to in a review of abnormal urine color a decade later.⁴ Early speculation about mechanism was replaced by detailedchemical analysis in 2 sentinel papers by Dealler and associates.^5,6 These showed that tryptophan is metabolized in the intestine to indole, which then ascends the portal vein and is conjugated in the liver to indoxyl sulfate, a species of indican. Indoxyl sulfate is excreted, considerably concentrated, into the urine. There, high concentrations of viable bacteria, if they possess the requisite sulfatase/phosphatase enzyme, can cleave the molecule. Following further chemical reactions, indigo (the familiar dark blue that is synthesized by the plant of the same name and, in the industrial era, in factories) can form, along with a violet pigment called indirubin. The latter in particular has an affinity for plastic and is incorporated into the tubing and bag. This combination of blue and violet stains the bag purple.

BACTERIOLOGY, EPIDEMIOLOGY, AND THE BOWEL

Providencia stuartii is the bacterium most commonly isolated from persons with purple urine bags, followed by Klebsiella pneumoniae and Enterobacter agglomerans. Ingenious studies have looked at the formation of blue bacterial colonies on conventional agar plates in a variety of conditions, including the addition of the filtered urine of affected persons. Careful microbiologic work has shown that the requisite enzymes are most common and most abundant in these species; however, other investigators find a several-log higher mean urinary bacterial count the most strongly predictive factor, rather than the presence of one particular organism.

Providencia stuartii, formerly classified asa Proteus species, is unfamiliar, yet it has proved to be a formidable and often multiply antibiotic-resistant cause of infection and persistent colonization, particularly in urethral catheter systems in nursing home settings.⁷ So although some cases of purple urine bags syndrome have been successfully managed with a change of the catheter and the bag, thus removing a principal reservoir, many others have necessitated systemic antibiotic therapy.^8-11 Surely for an organism that is often multiply resistant, one would prefer the earliest possible warning, regardless of which management decisions are ultimately made and the fact that Providencia stuartii has a color indicator, even a poorly sensitive one, sometimes provides just that.

Some readers will recall an indican test for malabsorption. Although this is no longer used because of insufficient sensitivity and specificity, there is actually a connection with the syndrome in question: the constipation that is a usual part of the setting of purple urine bags syndrome results in increased bacterial metabolism of tryptophan in the intestines of patients so affected, compared with persons who have normal intestinal transit time; hence, there is more substrate from which indoxyl sulfate can be derived. One could speculate on whether the use of l-tryptophan as a dietary or "nutraceutical" supplement, a use that fell sharply in the wake of its association with the eosinophilia-myalgia syndrome, would have led to additional cases of purple urine bags syndrome because of enhancement of urinary concentration of indoxyl sulfate. However, persons who are free of the requisite bacteriuria should have been immune regardless of the concentration of this chemical in the bladder urine.

WHAT ELSE TURNS THE URINE PURPLE?

One unique aspect of purple urine bags syndrome is the permeation of the pigment into the plastic bag. Of course this makes sense if one envisions the plastic bag and tubing--the collection system--as a conduit exposed to a substantial volume of urine en route from the kidneys to being flushed, during the lifetime of the disposable plastic element. Any substance dissolved in the urine that has an affinity for plastic can be, de facto, selectively extracted, and thereby become either more concentrated and hence more visible, or simply more conspicuous when separated from other urinary pigments.

Figure 2. Here the urine has faded sufficiently so that one is not sure if it is merely dark and concentrated or actually purplish, but a distinct purple tinge remains in the plastic on the left side of the image. (Courtesy of Alain Jégo, MD)

The myriad causes of abnormal color of the urine (not precisely what we see here, which is more the bag than the urine) were collected by 2 nephrologists in 1988. For purple, they mention porphyrins, especially under UV light; chlorzoxazone); and, in the presence of alkalinization, phenolphthalein, well known both as an acid-base indicator and as a bowel stimulant. The more familiar red color provided dramatic proof of laxative abuse that had been denied: add a quantity of base to a clear container of the patient's urine, in which the surreptitiously ingested laxative must have been excreted, and watch it turn red! The withdrawal of phenolphthalein from the market because of concerns about carcinogenesis has made the practice of medicine a little less vivid than it used to be. The list of causes of red, red-orange, and red-to-pink urine, which far exceeds the "blood and infection" that would come to mind instantly, remains worth a perusal.⁴

Purple urine bags syndrome continues to be reported because it has not yet become familiar. Studies that claim a 10% prevalence in patients with long-term urethral catheters seem discordant from the individual experience of many physicians, this one included, but nurses may note it more often. I mentioned purple urine bags syndrome to the nurses when cultures from a patient with non-purple urine and bag showed more than 100,000 colonies per mL of Providencia stuartii. The response was, "Yes, I saw that in a young paraplegic patient who had a catheter and frequent urinary tract infections, but I never knew what it was."

The science now exists for us to connect the otherwise confusing pieces of this puzzle, and to understand why the changing of tubing and the administration of appropriate antibacterials, when indicated, will lead to fading (Figure) and then disappearance of the extraordinary discoloration. *

Schneiderman H. Purple urine bags syndrome: another complication of chronic urethral catheterization. CONSULTANT. 2006;46:787-792.

(Courtesy of Alain Jégo, MD)