The Potential Clinical and Economic Benefits of Silver Alloy Urinary Catheters in Preventing Urinary Tract Infection FREE

WORLDWIDE, 96 million indwelling urethral catheters are sold annually; 24 million of these are sold in the United States. Approximately 25% of hospitalized patients have a urinary catheter placed at some time during their stay.¹ The incidence of bacteriuria in catheterized patients is approximately 5% per day.¹ Symptomatic urinary tract infection (UTI) occurs in approximately 20% of those with bacteriuria,^2- 3 with bacteremia complicating bacteriuria in up to 4% of patients.^4- 5

Given the morbidity and costs associated with catheter-related UTI,⁶ preventive measures have been attempted, including the addition of silver coating to urinary catheters. A recent meta-analysis⁷ concluded that silver alloy catheters significantly reduced bacteriuria by 3-fold compared with standard, noncoated catheters. Clinicians must decide whether the improved efficacy of silver alloy urinary catheters is worth the extra per unit cost of approximately $5. To assist decision makers in evaluating these catheters, we used decision-analytic techniques to assess the incremental clinical and economic impact of using urinary catheters coated with silver alloy in hospitalized patients requiring catheterization.

We compared 2 catheterization strategies: silver alloy catheters and standard, noncoated, urinary catheters (Figure 1). The hypothetical cohort in the decision-analytic model consisted of patients admitted to hospitals on general medical, surgical, urologic, and intensive care services requiring the short-term use (2-10 days) of indwelling urethral catheterization. We chose this cohort because most patients in the clinical trials evaluating the effectiveness of silver-coated catheters included these populations, and these patients are the primary recipients of indwelling catheters in clinical practice. A catheter duration of 2 to 10 days was chosen because most of the trials excluded catheters in place for less than 1 day and the average duration of catheterization in the trials was approximately 7 days (range, 3-21 days). The analysis was performed from the perspective of the health care payer, and the time horizon was defined as the period of hospitalization. Decision-analytic software (DATA 3.5; Treeage, Williamstown, Mass) was used for all analyses.

The probabilities and ranges of clinical events used in the decision model are shown in Table 1. The baseline risk of bacteriuria over the hospital stay in the control group (patients not receiving systemic antimicrobial agents and given short-term standard noncoated catheters) was statistically pooled from several prospective studies.^8- 17 The probability of developing a symptomatic UTI and bacteremia once bacteriuria developed was derived from the published literature.^{2- 5,18- 19} We assumed that the remaining 72% of patients with asymptomatic bacteriuria did not experience any adverse clinical sequelae or incur costs.

The probability of developing bacteriuria was dependent on the concomitant use of systemic antimicrobial agents and the catheter type. Approximately 80% of hospitalized patients receive antibiotics and are thus at lower risk for UTI.^12,20- 21 The relative risk reduction of bacteriuria (or protective effect) associated with antimicrobial use of 65% was derived from 5 studies^2,9,11,13,20 that adjusted for several factors (eg, sex and duration of catheterization) that affect the likelihood of developing bacteriuria. Similarly, the 45% relative risk reduction (or protective effect) associated with silver catheter use came from a meta-analysis of 5 randomized controlled trials. The probability of bacteriuria in patients with silver catheters who were not receiving systemic antimicrobial agents was a function of the relative risk reduction of silver catheters and the baseline probability of bacteriuria in the control group. The probability of developing bacteriuria for those taking systemic antibiotics was calculated in an analogous manner.

The cost estimates used in the model are shown in Table 1. The additional cost of a silver alloy catheter compared with a standard catheter is $5.30 (Marjory Greenhalgh, BA, at C. R. Bard, Inc, Covington, Ga, oral communication, July 2000). Urinalysis, urine culture, and blood culture costs were derived by converting the Current Procedural Terminology–based charges attributed to each of these tests to costs via The University of Michigan Health System's Medicare cost-charge ratio of 0.57. Hospital room costs were derived by multiplying the number of excess hospital days by per diem room costs after estimating the increased hospital days attributed to symptomatic UTI and bacteremia from literature reports.^22- 24 Antimicrobial costs were derived from The University of Michigan Health System pharmacy, Ann Arbor. Costs in 1998 US dollars are reported.

The estimated cost of a symptomatic UTI included the cost of urinalysis, urine culture and sensitivity, 7 days of therapy with a combination of oral trimethoprim and sulfamethoxazole, and one half extra day on the hospital ward. For patients with symptomatic UTI receiving systemic antimicrobial agents, oral norfloxacin was used rather than trimethoprim-sulfamethoxazole since these patients potentially could have acquired an organism resistant to trimethoprim-sulfamethoxazole. The added cost per case of symptomatic UTI for those not already taking systemic antimicrobial agents was $374; for those already taking systemic antimicrobial agents, it was $402. The estimated cost of bacteremia included the cost of urinalysis, urine culture and sensitivity, 2 sets of blood cultures, 3 days of intravenous ampicillin and gentamicin sulfate treatment, 11 days of oral trimethoprim-sulfamethoxazole therapy, and 3 extra days on the hospital ward. Thus, the estimate used in the base-case analysis for the incremental cost of bacteremia was $2041.

The outcomes estimated using the decision-analytic model for 1000 hospitalized patients requiring short-term catheterization were the incidence of symptomatic UTI and bacteremia and direct medical costs. To determine the effects of specific inputs on the results, we performed a series of 1-way sensitivity analyses using the ranges in Table 1. Notably, the low estimate for the incidence of symptomatic UTI was 8% and that for the incidence of bacteremia was 0%. In addition, the low end of our cost estimates included no extra days of hospitalization due to symptomatic UTI and only one extra ward day due to bacteremia.

To evaluate the impact of the uncertainty in all of the variables simultaneously, we performed a multivariate sensitivity analysis using Monte Carlo simulation.²⁵ Such a calculation provides an estimate of overall uncertainty by simulating the use of 10,000 catheters, where the clinical probabilities and costs are randomly drawn from probability distributions that represent the uncertainty of each of the variables.

In the base-case analysis, use of silver-coated catheters led to a 47% relative decrease in the incidence of symptomatic UTI from 30 to 16 cases per 1000 patients (number needed to treat = 74) and a 44% relative decrease in the incidence of bacteremia from 4.5 to 2.5 cases per 1000 patients (number needed to treat = 500) compared with standard catheters. In addition to these clinical advantages, use of silver-coated catheters resulted in an estimated cost savings of $4.09 per patient compared with standard catheter use ($20.87 vs $16.78).

The 1-way sensitivity analyses revealed that the silver-coated catheter strategy remained dominant throughout the ranges evaluated in Table 1. We determined threshold values for the following variables since they were particularly influential in the 1-way sensitivity analyses: baseline incidence of bacteriuria in the control group, probability of developing symptomatic UTI after bacteriuria, efficacy of silver-coated catheters, and cost of a silver-coated catheter. The probability of developing bacteriuria in patients receiving short-term catheterization with standard catheters and not receiving systemic antimicrobial agents would have to be less than 15% for the silver-coated catheters to not be cost saving. Similarly, the probability of symptomatic UTI (without bacteremia) after developing bacteriuria would have to be less than 5% for the silver-coated catheters to no longer provide cost savings. At a relative risk reduction associated with silver-coated catheters of 25% or greater (relative risk, ≤0.75), the clinical and economic superiority of the silver-coated catheter strategy persists. Finally, the cost of a silver-coated catheter would have to average $9.40 more than a standard catheter for silver-coated catheters to not be cost saving. Threshold estimates for all pertinent variables are outlined in Table 2.

The 95% central range of values obtained from the Monte Carlo simulation for silver-coated vs standard catheters revealed a reduced incidence of symptomatic UTI (5-31 cases per 1000 catheterized patients), a reduced incidence of bacteremia (0.8-4.5 cases per 1000 catheterized patients), and a cost difference ranging from a savings of $17.22 to an increase in overall cost of $3.19 per catheterized patient. In the simulation, silver-coated catheters provided clinical benefits over standard catheters in all patients and cost savings in 84% of patients.

Since up to 80% of patients with a nosocomial UTI have an indwelling urinary catheter,⁵ the best prevention strategy would be to avoid catheterization. However, indwelling urinary catheters are often necessary in hospitalized patients. Because of the high incidence and substantial morbidity associated with nosocomial UTI, silver-coated catheters have been developed to prevent catheter-related UTI in those requiring catheterization. The results estimated by our decision model indicate that using silver-coated catheters for patients requiring short-term urinary catheterization reduces the incidence of symptomatic UTI and bacteremia, and is likely to produce cost savings compared with standard catheters. This combination of clinical benefit and likely decreased costs makes silver alloy urinary catheters attractive for routine use in certain hospitalized patient populations requiring short-term catheterization.

The uncertainty of several of the variables used in our analysis merits discussion. The estimate of symptomatic UTI without bacteremia was based on the only 2 relevant prospective studies we could locate; the point estimates from these studies were 12%² and 32%.³ We further broadened the range of estimates used in the sensitivity analyses to include an even lower incidence of symptomatic UTI than reported in the literature. The threshold value for incidence of symptomatic UTI was less than half of the lower of these 2 estimates. Similarly, the attributable costs of symptomatic UTI and bacteremia in this patient population are not clear. We thus used a broad range for our sensitivity analyses, including rather low estimates for each. Encouragingly, our threshold analyses indicated that the cost of an episode of symptomatic UTI or bacteremia would have to be a fraction of the base-case estimates for each ($87 and $17, respectively) for the silver-coated catheter strategy to no longer provide cost savings. In addition, we were conservative by not including the extra costs associated with physician or nursing time and routine blood tests (complete blood cell count or chemistry panels) and by only including the minimum number of laboratory tests and antimicrobial regimens required to diagnose and treat symptomatic UTI or bacteremia in a patient. Finally, we used charges to derive costs, which are admittedly only estimates of the actual opportunity costs. We are reassured that the range of costs we used for a day on the inpatient ward, for example, includes the daily cost of an inpatient day at other academic medical centers, including the University of California at San Francisco Medical Center ($550) (S. Flanders, MD, written communication, February 11, 2000) and the University of Washington Medical Center, Seattle ($375).²⁶

The efficacy of silver-coated catheters was based on the results of 5 randomized trials, only one of which was performed in the United States and was published in abstract form.²¹ In a recently published meta-analysis,⁷ we found that silver alloy catheters were effective in reducing bacteriuria while silver oxide catheters were not. We statistically pooled the results of the most recent trial²¹ evaluating silver alloy catheters with the results of the 4 previous silver alloy trials^14- 17 included in the meta-analysis. It was previously demonstrated that estimates of effectiveness derived from meta-analysis are usually appropriate for use within cost-effectiveness analyses.²⁷ Encouragingly, 2 separate before-and-after studies recently performed in the United States found that the reduction in catheter-related UTI (40%²⁸ and 55%²⁹) is similar to the 45% risk reduction we estimated.

All of the trials included in our analysis used bacteriuria as an outcome measure rather than symptomatic UTI or bacteremia. Studies evaluating these latter outcomes would be helpful in more precisely determining the ultimate clinical benefit of silver catheters, but will require many patients. In fact, we believe that ideally a large, multicenter trial randomizing patients to either silver alloy or standard noncoated catheters be performed to confirm our findings. Such a clinical trial should have a formal economic analysis conducted alongside, and should be adequately powered to detect a clinically significant decrease in symptomatic UTI, not just bacteriuria. Such a trial would require 1939 patients in each arm to have 80% power to detect the reduction in symptomatic UTI incidence that we have estimated. Similarly, 14,669 patients in each arm would be required if catheter-related bacteremia becomes the primary outcome of interest. Given the sample size estimates, however, such a study would be expensive and time-consuming. A benefit of decision analysis is to provide estimates of clinical effectiveness by modeling clinical outcomes using established surrogate measures, such as bacteriuria. Thus, until the data outlined become available, our results suggest that silver-coated urinary catheters are likely to provide clinical benefits and cost savings for hospitalized patients requiring short-term catheterization over a reasonable range of cost and effectiveness variables.

The results of our study are not generalizable to all patients requiring indwelling catheters. The use of silver-coated catheters in those requiring less than 2 days of catheterization (eg, peripartum or some postoperative patients) and in those requiring long-term catheterization (eg, patients with injured spinal cords) may not lead to cost savings. Furthermore, it is unlikely that physicians will be able to determine at the time of hospital admission which patients are likely to be catheterized between 2 and 10 days. Thus, to most effectively use this new catheter, health care systems should estimate the average duration of catheterization for patients admitted to the different hospital wards, providing either silver-coated or standard catheters. This type of evaluation at the University of Michigan Medical Center revealed that most indwelling catheters were used between 2 and 10 days in all catheterized adult patient groups except those in the obstetric wards and patients undergoing some surgical procedures. Thus, one potential strategy would be to stock silver-coated catheters throughout the hospital except on the obstetric floor and in the operating room. Further studies are needed, however, to more clearly identify patient groups likely to derive the greatest amount of benefit from these catheters.

Until definitive evidence about the clinical and economic consequences of silver alloy catheters vs standard catheters becomes available, decision makers must decide whether to begin the routine use of these new catheters in their hospitals. Our decision-analytic model suggests that silver alloy catheters are likely to produce clinical and economic benefit, an unusual and attractive combination. Several well-studied methods of preventing catheter-related UTI have been previously proved to be useless, including irrigation of the bladder, instilling antimicrobial agents in the drainage bag, and rigorously cleaning the meatus.³⁰ Given the difficulties in preventing catheter-related UTI and the potential clinical and economic benefit associated with silver alloy catheters, this relatively simple intervention should be strongly considered in the appropriate clinical settings.

Accepted for publication March 8, 2000.

This study was supported in part through a grant from the Association of Practitioners of Infection Control Research Foundation, Washington, DC; a postdoctoral fellowship from Roche, Palo Alto, Calif (Dr Veenstra); and grants HS/HL08368-01A1 and HS07834-03S1 from the National Institutes of Health, Bethesda, Md (Dr Sullivan). Dr Saint was a Robert Wood Johnson Clinical Scholar at the time much of this work was conducted.

Presented in part at the 17th Annual VA Health Services Research & Development National Meeting, Washington, DC, February 24-26, 1999; and at the 22nd Annual Society of General Internal Medicine National Meeting, San Francisco, Calif, April 29-May 1, 1999.

Corresponding author: Sanjay Saint, MD, MPH, Division of General Medicine, Department of Internal Medicine, University of Michigan, 3116 Taubman Center, Campus Box 0376, Ann Arbor, MI 48109-0376 (e-mail: saint@umich.edu).