This study demonstrated that nearly a quarter of ureteral stones passed spontaneously within four weeks of stent placement. In multivariable analysis, stone density < 1000 HU, stone location in mid- to distal ureter or at the ureterovesical junction, and maximum diameter were independently associated with SSP. These findings formed the basis of the PASS score, a practical clinical tool for predicting SSP and guiding individualized treatment management.

The SSP rate observed in our cohort (24.2%) aligns with prior studies, which reported rates ranging from 8 to 26%. For instance, Nogara et al. reported a 26.2% SSP rate among 249 patients with stented ureteral stones, assessed by follow-up CT one month postoperatively [10]. In contrast, Kuebker et al. recorded a total SSP rate of 8% among 209 patients undergoing secondary ureteroscopy after median 21 days [11]. In addition to the shorter observation period, the distribution of stone diameter and location could be the reason for the lower SSP rate. In said study, 40% of all stones were \(\ge\) 7 mm in size and 75% were located in the proximal ureter. These rates were lower in our cohort with 31% and 38%, respectively. Interestingly, while unstented patients with ureterolithiasis < 5 mm often achieve SSP rates of up to 75%, our cohort exhibited significantly lower rates with an indwelling ureteral stent [12]. A similar divergence was observed in ureterolithiasis affecting the distal ureter, with respective passage rates of 75% without and 38% with inserted stent [12]. The presence of an indwelling ureteral stent appears to obstruct the natural passage of stones through the ureter despite its passive dilatation. The stent reduces the lumen available for the stone to pass through and also alters the natural flow of urine. In addition, the stent may physically interact with the stone, causing it to become trapped or restrict its movement. These mechanical factors may explain the higher passage rate in unstented patients.

Surprisingly, in multivariable analysis we extrapolated stone density > 1000 HU as the strongest predictive factor for SSP next to ureteral stents, followed by the location at the ureterovesical junction and maximum stone diameter (Table 3). This finding corroborates prior studies, which have shown that lower stone density is a significant predictor of SSP. For example, Nogara et al. identified increasing stone density as a continuous variable inversely associated with SSP probability [10]. The pronounced effect in our study may be attributed to the dichotomous classification of density, with < 1000 HU as the threshold.

Coşkun et al. also found lower stone density as an independent predictive factor for SSP in multivariable analysis in unstented patients [13]. In a similar study, low stone density was a significant predictor for SSP in univariable analysis [14]. Eliseu et al. showed a higher SSP in radiolucent calculi in conventional imaging [15]. Interestingly, the majority of patients in our cohort with low-density ureterolithiasis did not notice SSP and therefore their stones could not be tested for chemical composition.

We hypothesize that low density stones might fragment more easily due to frictions with the stent, facilitating fragment passage. Additionally, some low-density stones, particularly those composed of uric acid, may partially dissolve over time. While the exact mechanisms remain unclear, our findings underscore the clinical importance of assessing stone density in predicting SSP.

Stone location within the ureter also significantly influenced SSP rates. In our study, stones at the ureterovesical junction had the highest SSP rate (45.5%), followed by distal and mid-ureteral stones. Proximal stones, in contrast, exhibited minimal likelihood of passage. These results are consistent with prior literature, which has identified distal stone location as a key determinant of SSP [11, 16, 17].

Regarding the stone location, various definitions can be found in the literature. While the concept of three physiological narrowing is widely used, some authors have stated that there are only the proximal ureter and the ureterovesical junction as sites of obstruction [18]. With this in mind, stones located at the ureterovesical junction were identified and analyzed separately in our cohort (Fig. 1). In the backward selection process, location at the ureterovesical junction was the strongest predictor of SSP and was therefore used as the only factor in the final prediction model. The iliac crossing classification was found to be inferior and was not included in the final model.

As expected, smaller stone size was strongly associated with SSP. Stones ≤ 5 mm were nearly 3.5 times more likely to pass compared to larger stones. This observation is consistent with previous studies, which have uniformly demonstrated an inverse relationship between stone size and passage rates. Importantly, our PASS score assigns greater weight to stone location than size, reflecting the relative contributions of these factors to SSP probability (Table 3).

The PASS score provides a simple, evidence-based tool for estimating SSP probability in stented patients. By integrating stone density, location, and size, the score facilitates individualized treatment planning. To the best of our knowledge, this is the first study that offers a factor-weighted predictive model that provides the probability of SSP for an individual stented patient.

Stojkova et al. evaluated stone passage within 24 h after ureteral stent removal, 61% of ureteral stones passed spontaneously [19]. Leventhal and Deliveliotis found SSP rates of 83% and 85% respectively, two weeks after stent removal [20, 21]. This leads to the assumption, that stones might be expulsed by the stent ablation or SSP seems to occur more frequently shortly after stent removal, presumably due to ureteral dilatation. Strategies such as stent removal without immediate ureteroscopy could be particularly beneficial in healthcare systems with limited resources.

Clinical implications of the PASS-Score: Patients with a high PASS score (PASS Score ≥ 6) may benefit from a confirmatory CT scan prior to ureteroscopy to ensure stone persistence and potentially allow for direct stent removal, as stone passage is more likely after stent removal, promoting spontaneous expulsion [19,20,21]. In contrast, a low PASS score (PASS Score < 6) may justify omitting CT, reducing radiation exposure and costs. These patients may benefit from timely secondary ureteroscopy to avoid prolonged stenting and its associated complications. However, external validation is needed before clear clinical implications can be defined.

Several limitations should be acknowledged. This study was retrospective, introducing potential biases. While the large cohort and standardized follow-up protocol are strengths, the lack of external validation are limitations. External validation is essential before recommending the PASS score for clinical application. Additionally, the stenting duration in our cohort was uniform, precluding analysis of its potential impact on SSP rates.

Regarding the density value measured in HU, no clear threshold value could be found in the literature. Due to the continuous nature of the density values in the cohort, the selection of the cut-off value was based on the distribution of the data only. Other radiological findings, such as ureteral wall thickness that might be associated with impaction of stones were not assessed.

Future studies should evaluate the performance of the PASS score in external populations and explore the role of stent removal as a standalone intervention.