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Urinary catheters

Apparently about (deliberate Welshism - Ed) one hospital patient in four has an indwelling urinary catheter. Urinary catheters are associated with urinary tract infection in about 5% of those with an indwelling catheter, and with bacteraemia also in some of these patients. Bandolier calculates from these figures that out of every 1000 patients in hospital, about 12 will have urinary tract infection because of their indwelling urinary catheter.

What can be done to reduce this? Work has been done to give the catheters themselves antimicrobial properties. This has included incorporating antibiotic drugs onto the surface of the catheters using chemical methods, and using silver-coated catheters, because of the antimicrobial actions of silver. The idea is that surfaces with antimicrobial properties (which may be on the inside of the catheter, or on the outside, or both) will inhibit the growth of nasty bugs and so prevent health problems occurring.

Do they work? A meta-analysis [1] of silver catheters says yes and no, depending on the type of silver coating employed.

Process issues

The searching was with MEDLINE, not limited to English, and other ways of obtaining trial data were pursued, including grilling the manufacturers (not literally, just asking questions about what trials they had or had not done). To be included trials had to have compared a silver-coated catheter with an uncoated catheter. The outcome chosen was bacteriuria as judged by urine culture. No studies were analysed in the meta-analysis in which patients were bacteriuric at baseline, or where there was an open urinary drainage system.

Eight trials were identified, four using silver alloy coating both inside and outside the catheter, and four using silver oxide coating, two of which had the coating on both internal and external surfaces, while two had external coating only. The patient groups were mixed, with urology, surgical and medical patients being included in the various trials. Definitions of bacteriuria used in the trials varied. All were above 200 colony forming units per mL (two studies); two were at 1000 and four were at 100,000 colony forming units per mL.


Six of the studies were randomised by individual patients. Two allocated patients by week or month. Neither investigators nor patients were blinded to type of catheter used in any study.

For silver alloy catheters, 11% of patients had bacteriuria, compared with 32% with uncoated catheters (Figure).
The number needed to treat with a silver-alloy coated catheter to prevent one case of bacteriuria was 4.6 (Table).

Efficacy of silver-coated urinary catheters
  Number (%) of patients with bacteriuria    
  Silver-coated Uncoated Relative risk (95%CI) NNT (95% CI)
Alloy 23/216 (11) 77/237 (32) 0.32 (0.21 to 0.49) 4.6 (3.4 to 6.9)
Oxide 122/1019 (12) 123/883 (14) 0.82 (0.65 to 1.03) 51 (20 to no benefit)

For silver oxide catheters, 12% of patients had bacteriuria, compared with 14% with uncoated catheters (Figure). The number needed to treat with a silver-oxide coated catheter to prevent one case of bacteriuria was 51 (Table).


This is a fascinating report. There are many reasons to consider the validity of the results, which include:
  • Bacteriuria is a surrogate end-point, rather than overt urinary tract infection or catheter-associated bacteraemia, or death.
  • Methodological issues over randomisation, blinding, the use of prophylactic antibiotics, and gender (men and women may differ in their susceptibility to infections with urinary catheters).
  • The very variable rates of bacteriuria with control, from 10% to 55% in individual studies, and the effect of trial size, with larger studies having lower rates of control infection.
  • The overall rate of 14% with silver oxide controls and 32% for silver alloy controls.
  • The fact that all four silver alloy studies were done at the same institution by the same investigators.
Given some of these concerns, can we be sure of the results? Bandolier thinks caution is needed, as, to be fair, do the authors. But the clear and well-written nature of the report makes it ideal for anyone wanting to use it for critical appraisal work, and going on from there to design a trial which might prove the point without dispute.

There is also an interesting health economics perspective here. According to the authors, a silver alloy coated catheter costs twice that of an uncoated catheter in the USA (about US$7 difference). What are the health economic implications? It should be possible to examine how much better silver alloy catheters would have to be for them to be cost-effective, given some reasonable assumptions about rates of infection with uncoated catheters. And if the "back-of-stamp" health economics suggests that catheters with antimicrobial properties save money, then what trials do we need to prove it, and to get better practice implemented?

This paper makes you think, especially about issues like technology creep, where innovations may be small in their costs and consequences, but where the pennies add into many pounds. It also makes you think we should be doing more, and better, and faster, than we seem to be doing now.


  1. S Saint, JG Elmore, SD Sullivan et al. The efficacy of silver alloy-coated urinary catheters in preventing urinary tract infection: a meta-analysis. American Journal of Medicine 1998 105: 236-41.

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