#2827 Beyond the scale: unveiling the kidney stone risk after bariatric surgery
Authors: Vidal Helena , Fonseca Nuno Moreira , Jorge Cristina 
Published: May 2024

Abstract

Background and Aims
Nephrolithiasis is a prevalent condition, with calcium oxalate being the primary component found in kidney stones. Both obesity and gastric bypass surgery pose as risk factors for nephrolithiasis. Malabsorptive bariatric procedures alter 24-hour urine profiles, leading to increased urinary oxalate and reduced urinary citrate levels. This study aims to evaluate the risk factors for calcium oxalate lithiasis in a population after gastric bypass surgery at our center.

Method
We performed a monocentric retrospective study to evaluate hyperoxaluria and urinary lithiasis in patients who underwent gastric bypass surgery between 2020 and 2021. We assessed urinary oxalate excretion, urinary citrate excretion, and urinary sodium excretion between 4 and 28 months after surgery. To confirm the adequacy of the collection, a comparison was made with the expected urinary creatinine value (20 to 25 mg/kg ideal weight for men and 15 to 20 mg/kg ideal weight for women).
We developed a risk score: citraturia &lt; 288 mg/24 h (1 point); oxaluria &gt; 45 mg/L (1 point) and natriuresis &gt; 97.75 mmol/24 h (1 point). Patients were classified as without risk (0 points), at low risk (1 point), at high risk (2 points), and at very high risk (3 points). Additionally, the patients’ historical incidence of lithiasis was examined, both preoperatively and postoperatively, using clinical records. Post-surgery assessments included imaging evidence of lithiasis (when available) or clinical episodes of renal colic.

Results
Fifty-six patients underwent a 24-hour urine collection. Twenty-two were excluded due to incomplete urine collection, determined by the expected urinary creatinine value and a time lapse exceeding 28 months between surgery and analytical assessment. The median time elapsed between surgery and analytical evaluation was 9.5 months [IQR 6.3 - 12] months. A total of 34 patients were included in the study, 8.8% were male and the median age of the patients was 51.5 [IQR 42.3–57.8] years. The median body mass index decreased from 43.1 [IQR 39.7–46.3] kg/m2 before to 31.8 [IQR 28.6–35.7] kg/m2 six months after surgery.
The low risk for calcium oxalate lithiasis in the examined patient sample was 52.9% (n = 18); high risk 20.6% (n = 7), no risk 26.5% (n = 9) and very high risk 0%. We detected 3 (8.8%) patients with concentration hyperoxaluria, 20 (58.8%) patients with natriuresis &gt; 97.75 mmol/24 h and 8 (23.5%) patients with reduced urinary citrate levels. All patients exhibited normal kidney function.
The median follow-up duration was 27 months. Out of the total number of patients, only 3 reported renal colic episodes, and imaging evidence of lithiasis was found in just 2 of them before surgery. Among those with a risk score &gt; 0, only 1 had a history of lithiasis. Throughout the follow-up period, none of the patients experienced new lithiasis post-surgery.
Regarding the limitations of our study, we highlight the relatively brief duration of follow-up, which may pose challenges in comprehensively evaluating outcomes such as lithiasis. Additionally, the absence of imaging screening represents a notable constraint, impeding the comprehensive assessment of patients with asymptomatic lithiasis.

Conclusion
This study concludes that within the bariatric surgery population, a specific subgroup is at an elevated risk for renal lithiasis. Recognizing this subgroup promptly and instituting heightened preventive measures may prove advantageous within this demographic.