Calcium Oxalate Monohydrate and Dihydrate Stone Formers: Differing Surgical Recurrence Rates and Metabolic Profiles in a Large Cohort

Background: Calcium oxalate (CaOx) stones have a lower recurrence rate compared with other stone types. However, their high prevalence results in a substantial clinical and economic burden. Calcium oxalate monohydrate (COM) and dihydrate (COD) are the main CaOx stone subtypes. The clinical significance of the presence of COM or COD is not certain. This study aims to evaluate the surgical recurrence rates and metabolic profiles of different CaOx stone subtypes. Patients and Methods: A retrospective analysis of surgically treated patients with an available stone composition analysis performed between 2013 and 2022 in a large health care provider database. Data were analyzed for as much as 5 years from the initial surgery. Demographic, metabolic, and surgical characteristics were collected. Stones were classified based on their dominant component (>50%). Patients with COM stones were compared with those with COD stones. Results: The study cohort included 16,091 patients with stone analysis. Of these, 13,018 (80.9%) had CaOx stones, classified into COM (10,891; 83.7%) and COD (2127; 16.3%) groups. Compared with COM patients, COD patients were significantly younger ( p < 0.01) and had fewer comorbidities. COD patients exhibited significantly higher urine calcium levels (242 mg/day vs 156 mg/day, p < 0.01) and lower citrate levels (377 mg/day vs 402 mg/day, p = 0.03). Conversely, COM patients had higher sodium excretion (161 mg/day vs 144 mg/day, p < 0.01). The 5-year surgical recurrence rate was 47% higher in COD patients compared with COM patients (14.3% vs 9.8%; hazard ratio (HR) = 1.53, 95% confidence interval: 1.33–1.76, p < 0.001). Additionally, COD patients required more repeat surgeries on average (1.45 vs 1.34, p = 0.04). A dominant COD composition was a significant predictor for 5-year surgical recurrence (HR = 1.69, 1.38–2.07, p < 0.001). Conclusions: CaOx stone subtypes exhibit distinct metabolic characteristics and surgical recurrence rates. COD patients are more likely to experience surgical stone recurrence. Therefore, patients with a dominant COD stone composition may require a more comprehensive metabolic workup and closer follow-up.