Abstract
Our hypothesis about the black coloring was merely a hypothesis and not based on the basic research. Many urologists have experienced coloring (almost to black) of ureteral stents; however, few black urethral balloon catheters were seen. Therefore, we thought that the difference between ureteral stents and urethral balloon catheters was because of whether the materials were radiopaque or not. In addition, most of the patients whose ureteral stents are colored black have a tendency to have urinary tract infections. This led us to our hypothesis about the coloring. As the reviewer noted, however, we did not show the urinalysis and urine culture results. Because our institution is a third referral stone disease center in Japan, more than 400 shockwave lithotripsy (SWL), 300 ureteroscopy (URS), and 80 percutaneous nephrolithotomy (PCNL) procedures are performed per year, so some patients were treated with SWL or URS at other hospitals and their stones were resistant to these procedures. 3 Some had also undergone previous ureteral stent placement. To exclude the effects of whether ureteral stent placement had been performed or not and the various reasons for ureteral stent insertion, we did not perform a urinalysis for all of the patients. Despite these limitations, our study still fulfilled its purpose, which was to examine the encrustation, incrustation, coloring, and resistance to removal in relation to the indwelling time for a large-scale observational study.
As noted, however, urinalysis and urine culture are also important points when assessing the coloring of ureteral stents. Therefore, we again reviewed the clinical records and performed the analyses, including more patients whose stents were colored black. 1 We compared 26 black stents (heavily black) and 69 noncolored stents. To account for the indwelling time, we excluded the patients who had stents at the conclusion of ureteroscopic lithotripsy and did not need an additional procedure.
Therefore, a urinalysis and urine culture were performed for these 26 patients with colored ureteral stents (14 patients with stents and 12 patients without stents) and 69 patients with noncolored ureteral stents (22 patients with stents and 47 patients without stents). With regard to the urine culture, 25 of 26 (96.1%) patients with colored ureteral stents showed positive urine cultures and 50 of 69 (72.5%) patients with noncolored ureteral stents showed positive urine cultures. In the urinalysis, a high level of white blood cells (more than 20/high power field) were observed in 47.0% (colored ureteral stent) and 28.9% (noncolored ureteral stent) of the patients. These data showed that there was a tendency for the colored stents to be more frequently infected; however, the mechanisms responsible for the coloring and why not all infected patients exhibited coloring are still unknown.
As a preliminary study to uncover the mechanism responsible for the black coloring, we put a variety of ureteral stent and ureteral catheter fragments in the patients' urine bags and patients' urine. In a small number of cases (three in urine bags and four in urine), no coloring was found within 2 weeks. Further studies including clinical and basic research are needed to reveal the reasons for the ureteral stent coloring. We believe that revealing the mechanism will be an important factor that will help reflect the condition of the patient's urine.