The incidence and prevalence of upper urinary tract stones have been increasing globally (1–3). Common treatment methods for upper urinary tract stones include extracorporeal shock wave lithotripsy (ESWL), ureteroscopy, percutaneous nephrolithotomy (PCNL), and, in some cases, open surgery (4–6). With advancements in medical technology for endourological procedures, retrograde flexible ureteroscopy (R-FURS) for stone removal has become widely used. Its high efficacy and low complication rate in treating kidney stones up to 20 mm in diameter make it a preferred option for upper urinary tract stones (7).

The minimally invasive nature, precision, and low complication rate of retrograde ureteroscopy have expanded its indications (8). Although the number of PCNL procedures has remained stable over the past few decades, ureteroscopic procedures have significantly increased compared to ESWL (9). As an endoscopic surgery, retrograde ureteroscopy has fewer complications, but there are still some risks, with the complication rate around 10%–15%, most of which are Clavien grade II or lower (10). Urinary tract infection (UTI) and bleeding, as the most common complications, deserve particular attention (11).

Exploring the risk factors for UTI and bleeding after R-FURS for stone removal is crucial for identifying potential risks and taking effective preventive measures. This will help reduce the incidence of post-operative UTI and bleeding and provide valuable references for clinicians during diagnosis and treatment.

Most urinary tract stones require surgical treatment, with approximately 22% of upper ureteral stones passing naturally (12). R-FURS has become a widely applied, minimally invasive, and effective method for treating upper urinary tract stones (13). Although this method has achieved good results in treating upper urinary tract stones, postoperative complications still pose certain risks.

In the last two decades, advances in medical devices for endourological surgery have made R-FURS widely used, but the occurrence of complications has attracted more attention. Postoperative UTI and bleeding are the most common complications of RIRS. The incidence of significant postoperative bleeding is about 0.1%–2% (14), while mild bleeding, such as hematuria, occurs at a higher rate (10% to 20%) and is mostly self-limiting, not requiring special treatment (15). Omar et al. (15) found that 87.7% (64/70) of patients undergoing holmium laser lithotripsy for ureteral stones achieved stone clearance, with 4% experiencing mild bleeding. The postoperative UTI rate was 6.3%, and the sepsis rate requiring intensive care was 1.3% (14). In this study, the postoperative UTI rate was 8.15% (11/135), and the postoperative bleeding rate was 11.85% (16/135), which is consistent with previous studies.

Among PCNL, ESWL, and R-FURS, PCNL generally has a higher complication rate, followed by R-FURS, while ESWL has the lowest. However, in Kartal et al.'s study (16), no significant difference was observed in complication rates between R-FURS and ESWL in the first 15 days after surgery (p = 0.066); however, by the third month, significant differences were noted (p = 0.022). This difference was attributed to a higher complication rate in ESWL compared to R-FURS. For experienced surgeons, R-FURS may achieve a lower complication rate compared to ESWL due to its more precise operation, better stone removal, and less damage to surrounding tissues, while the high-energy shockwaves from ESWL can cause more damage to surrounding tissues, making it harder to avoid.

In this study, it was found that high preoperative creatinine levels are risk factors for postoperative urinary tract infection and bleeding. This may be related to the following factors: (a) An increase in creatinine usually indicates impaired renal function. When there is renal insufficiency, the body's metabolic products cannot be effectively excreted, and some substances that affect blood coagulation function, such as guanidines, phenols and other toxic substances, will accumulate. During the operation, it is prone to cause bleeding, and the wound surface is not easy to stop bleeding after the operation, increasing the risk of postoperative bleeding. (b) When there is renal insufficiency, the overall nutritional status of patients is often poor. The metabolic disorder of nutrients such as proteins will affect the normal function of the body's immune system, leading to the inhibition of the generation, differentiation and function of immune cells, a decrease in the level of immunoglobulins, and a weakening of the body's immune defense ability. Therefore, patients are more likely to be invaded by pathogens such as bacteria after the operation, increasing the probability of developing urinary tract infections. The assessment of preoperative renal function is, to a certain extent, beneficial for clinicians to judge the occurrence of postoperative urinary tract infection and bleeding.

There is currently no clear conclusion regarding the risk factors for the occurrence of complications after R-FURS. In this study, it was found that the size of the stone is an independent risk factor for infection after R-FURS, and the size of the stone and the operation time are independent risk factors for bleeding after R-FURS. The multivariate logistic regression analysis conducted by Shimpei Yamashita et al. (17) showed that female gender (p = 0.02) and the presence of multiple stones (p < 0.01) are independent and significant predictive factors for postoperative febrile urinary tract infection. The presence of multiple stones has been reported in multiple studies as a predictive factor for infectious complications in URS cases (18, 19). In this study, all the included patients had single stones, and the impact of multiple stones on postoperative urinary tract infection was not analyzed. In the study by Francesco Prata (20), it was found that the diameter of the stone, the number of stones, the type of ureteroscope, and the operation time are important predictive factors for postoperative urinary tract infection. The operation time and the type of ureteroscope are independent predictive factors for postoperative urinary tract infection. In the study by Peng et al. (21), it was found that gender, age, diabetes, stone diameter, urethral catheter insertion time, and operation time are independent risk factors for urinary tract infection after R-FURS. This is the same as the finding in this study that the size of the stone is a risk factor for infection after R-FURS. For larger stones and multiple stones, a longer operation time is required to break up or remove the stones during R-FURS. This will lead to increased irritation and trauma to the urinary tract and surrounding tissues, thus increasing the risk of postoperative bleeding and UTI. At the same time, when the stones are larger or there are multiple stones, their surfaces may carry bacteria or pathogens of UTI. During the operation, the rupture of the stones may cause these bacteria to enter the urinary tract, thus triggering an infection (22). Clinical guidelines recommend endoscopic intervention for kidney stones no larger than 20 mm in size and challenging lumbar stones (23, 24). This may also be due to concerns that overly large stones are likely to lead to the occurrence of postoperative complications.

There are relatively few studies on bleeding after R-FURS, and most of them focus on the analysis of the risk factors for bleeding after PCNL (25, 26). The study by Han et al. (27) found that for the treatment of kidney stones by PCNL, the independent risk factors for postoperative bleeding are multiple stones (p = 0.008) and stone size (p = 0.014). In the study by Tan et al. (28), severe bleeding after PCNL is associated with lower calyx puncture, multiple kidney stones, and single kidney stones. However, the general cause of bleeding in PCNL may be related to puncture. The bleeding after R-FURS is more closely related to the nature of the stones. In the study by Carlo Giulioni et al. (14), 6669 patients underwent R-FURS, and 5.5% of the patients required blood transfusion due to bleeding after the operation. The patients who needed blood transfusion generally had larger stone diameters, especially among those with stones exceeding 20 mm. In this study, it was found that stone size and operation time are independent risk factors for bleeding after R-FURS. There may be the following reasons. Larger stones require longer operation time and more complex operations, which not only increase the mechanical damage to the urinary tract but may also lead to urinary tract obstruction, fragment retention, and local inflammatory reactions, further increasing the risk of postoperative bleeding. Prolonged operation may lead to further damage to the urethral and ureteral mucosa, increasing the possibility of blood vessel rupture.

In fact, there may be many influencing factors for UTI and bleeding after R-FURS, which are not all covered in this article. The placement time of the ureteral stent before the ureteroscopy may be strongly correlated with UTI (29). The use of antibiotics also determines the occurrence of UTI to a certain extent. Factors such as the surgeon's operative experience and the choice of the endoscopic sheath during the operation are all related to bleeding. These may all be aspects that clinicians need to pay attention to.

Although the risk factors analyzed from different data sources are different, this still has clinical significance. It provides a certain reference for optimizing patient management, reducing complications, and improving the postoperative recovery effect. By understanding and addressing these risk factors, the treatment effect can be further improved, the incidence of postoperative complications can be reduced, and thus the safety and quality of life of patients can be enhanced.

This study has certain limitations. First, due to its retrospective design, some key indicators that may affect postoperative urinary tract infection and bleeding are missing, such as ureteral stent dwell time, antibiotic prophylaxis or treatment, and stone composition. May affect the comprehensiveness and accuracy of the results. Second, as a single-center study, the sample source is relatively homogenous, and regional bias may exist, limiting the external applicability of the results. In this study, the numbers of cases with infection and bleeding as the outcomes are relatively small. To a certain extent, there may be some bias in conducting statistical analysis. Furthermore, the retrospective nature of the data collection may lead to incomplete or inconsistent variable recording, which could affect the reliability of the conclusions.

Stone size is an independent risk factor for postoperative UTI after RIRS. Stone size and surgical time are independent risk factors for postoperative bleeding.