In this prospective study, data of 60 consecutive patients that underwent mini-PNL (n = 31) or RIRS (n = 29) and consented for the study between March 2019 and June 2019 was investigated. All of the procedures were performed by a single experienced surgeon with expertise of > 150 PNL and RIRS cases annually.
2.1 Inclusion and exclusion criteria
All of the patients were scheduled for active stone removal for stones < 3 cm in greatest diameter by mini-PNL or RIRS. The cut-off value of 3 cm was determined with respect to the institutional policy of performing RIRS for stones < 3 cm in diameter. All patients were evaluated with a non-contrast computerized tomography (NCCT) prior to surgery. The choice of the surgical method was decided by the surgical team with collaboration of the patients for their expectations without randomization. All of the patients were free of urinary tract infection verified by sterile urine cultures and all patients had normal preoperative serum creatinine levels. Patients with active UTI or elevated creatinine levels were excluded from the study for standardization. All patients underwent surgery for only kidney stones and none of the patients had concomitant ureteral stones. All patients included in the RIRS group were operated with proper placement of a ureteral access sheath. We excluded patients in whom access sheath could not be placed for the sake of standardization as the intrarenal pressure is the hypothesized surrogate for AKI in RIRS cases.
2.2 Surgical procedures
Mini-PNL cases were performed in Galdakao-Modified Supine Valdivia position. Initially cystoscopy was performed and a ureteral catheter was placed in the ureter and retrograde pyelogram was performed by injection of the radiopaque contrast material. Renal puncture was performed with aid of fluoroscopy and a hydrophilic guidewire was placed in the collecting system. The MIP-M kit (Karl Storz, Tuttlingen, Germany) was used for mini-PNL cases. One shot dilation with 15 Fr metallic dilator was performed and 16 Fr metallic sheath was placed. 12 Fr nephroscope was introduced and laser lithotripsy was performed in all cases. For active stone clearance vacuum cleaner effect was applied . Nephrostomy tube was not placed in any of the cases and a 6 Fr JJ stent was placed in all of the cases.
RIRS case were performed in lithotomy position. A hydrophilic guidewire was placed in the ureter and a 9.5/11.5 Fr ureteral access sheath (Cook, Flexor®, Bloomington, IN, USA) was introduced over the guidewire under fluoroscopic guidance. A fiberoptic flexible ureterorenoscope (FLEX-X2, Karl Storz, Tuttlingen, Germany) was used and stones were fragmented by holmium laser. The main strategy was to dust the stone to tiny particles that can pass spontaneously and in case of small fragments active stone retrieval was also performed by a nitinol basket. A JJ stent was placed in all of the cases following the procedure. Irrigation during both PNL and RIRS procedures was performed with gravity and no active manual or automized pumping system was applied. The height of the saline bag was fixed in less than 50 cm.
At the beginning of the procedure cystoscopy was performed in all patients for ureteral catheterization and the baseline urine samples (0 h) for measurement of NGAL levels were collected through the cystoscope. The second urine samples were collected at the postoperative 6th hour. The urine samples were stored at—80 °C until analysis. NGAL levels were measured by the chemiluminescence microparticle immune assay using the Abbot Architect i1000 immunology analyzer with a commercially available kit (Abbott Ireland Diagnostic Division, Sligo, Ireland).
The other parameters collected were age, gender, stone diameter, duration of operation, duration of hospitalization, perioperative complications, SFR, and auxiliary procedures. Residual fragments were evaluated with ultrasonography or KUB prior to JJ stent extraction and in cases of suspicious residual fragments an NCCT was also performed. Stone free was defined as the absence of residual fragments of any size in the postoperative imaging. Complications were recorded according to Clavien Classification.
2.3 Sample size calculation
The primary end point of the study was to compare the changes of NGAL levels and sample size calculation was performed according to this parameter. In the previous studies NGAL levels showed an increase of 50% for PNL cases6 and 30% for ureterorenoscopy  postoperatively. Sample size calculation revealed that 28 patients were needed for each group to determine a difference of 20% increase in NGAL levels with an alpha value of 0.05 and power level of 80%.
2.4 Statistical analysis
Statistical analysis was performed with SPSS for Windows, ver. 22.0 (SPSS Inc., Chicago, Illinois, USA). The normality of the data was evaluated with Kolmogorov–Smirnov test. The Student’s t test or Mann–Whitney U test was used to compare the continuous variables according to the normality of the data. Categorical variables were compared with the Chi-square test. Paired sample t test was used to compare preoperative and postoperative NGAL levels in each group. For statistical significance p value of 0.05 was accepted.