Renal Stents

Predictive value of urinary cultures in assessment of microbial colonization of ureteral stents

BACKGROUND AND PURPOSE

Bacterial colonization of indwelling ureteral stents may serve as a nidus for bacteriuria in operations where stents are manipulated. The predictive value of urine cultures in the assessment of stent colonization was examined in 65 patients with indwelling ureteral stents.

PATIENTS AND METHODS

Prophylactic antibiotic treatment was administered prior to stent insertion. All patients were ambulatory at the time of investigation and were examined in the outpatient clinic. Urine cultures were taken prior to stent removal after 8 to 150 (mean 64) days. The stents were removed under aseptic conditions, and the proximal and distal ends were cut off and placed in a culture medium for evaluation. None of the patients was treated for urinary tract infection prior to stent removal.

RESULTS

Bacteriuria was found in 15% of the patients. In 35 patients (54%), urine and stent cultures were sterile. In 20 patients (31%), the urine culture was sterile but the stent was colonized (Enterococcus 9, E. coli 5, Staphylococcus aureus 2, S. epidermidis 2, Candida 1, Citrobacter diversus 1). One patient had a sterile stent culture with bacteriuria. In 9 patients (13.5%), urine and stent cultures were identical (E. coli 4, Pseudomonas 4, Candida 1). The incidence of stent colonization did not correlate with stent dwelling time. The sensitivity of urine cultures for the detection of stent colonization was poor, being 31% only. In a specific patient with negative urine culture, the probability of stent colonization was 36%.

CONCLUSION

A sterile urine culture does not rule out the stent itself being colonized. Therefore, patients with indwelling ureteral stents and a sterile urine culture may benefit from prophylactic antibiotic treatment prior to endourologic procedures. The prophylactic regimen must provide coverage for common gram-negative uropathogens as well as gram-positive bacteria, including enterococci.

Mechanical performance of polyurethane ureteral stents in vitro and ex vivo

A serious problem associated with the use of ureteral stents is fracture in situ. Following clinical observations of fracture of polyurethane stents in vivo, this study examined the mechanical properties of 17 polyurethane stents (double-J containing drainage holes) retrieved from patients over a 24-week period of insertion. In addition, stents were immersed in human and artificial urine in an in vitro model at 37 degrees C to determine their general propensity to fracture. Mechanical properties of ureteral stents were examined using the standard ASTM D-412 tensile test and by the novel application of dynamic mechanical analysis (DMA). The ultimate tensile strength and elongation at break (but not the Young's modulus) of unused polyurethane stent sections containing side-drainage holes were greater than stent sections devoid of side-drainage holes. No correlations were observed between increased or decreased Young's modulus, ultimate tensile strength or elongation at break of polyurethane stents and their time of immersion in either human urine or artificial urine in simulated upper urinary tract conditions of 37 degrees C and 5% CO2. Similarly, no correlations were observed between Young's modulus, ultimate tensile strength or elongation of polyurethane stents and stent dwell time in situ. DMA of retrieved stents revealed that their tan delta value and storage modulus did not differ significantly from unused stents following dwell times in situ of up to 24 weeks. No changes in the glass transition temperatures were observed in retrieved stents. Although patient variation was observed, the results indicate that the polyurethane stents examined in vitro and following removal from patients did not exhibit any greater propensity to fracture than their unused counterparts. Fracture of retrieved polyurethane stents, arising in vivo and also during subsequent tensile testing, was observed to occur along the drainage holes, suggesting that elimination of these holes will reduce the incidence of polyurethane ureteral stent fracture in use.