MP75-02 DO PATIENT AND STONE FACTORS DIFFER BETWEEN CALCIUM PHOSPHATE STONES AND OTHER METABOLIC STONES UNDERGOING PERCUTANEOUS NEPHROLITHOTOMY?

Abstract

INTRODUCTION AND OBJECTIVES: Stone composition of large and complicated stones has shifted from predominantly infectious to metabolic. Calcium phosphate is emerging as the predominant composition of these metabolic stones. The etiology of this shift is unknown. We determined the incidence and compositions of metabolic stones undergoing percutaneous nephrolithotomy (PCNL). We then looked to see if patient and stone factors differed between metabolic stones that were predominantly calcium phosphate and other metabolic stones. METHODS: We conducted a retrospective review of patients who underwent PCNL at 2 academic institutions between 2002 and 2014. Stone composition, patient factors and stone characteristics were reviewed. Stones were characterized as infectious if any component of struvite or carbonated apatite was present. Metabolic stones were classified by the largest component present greater than or equal to 50%. All metabolic stones were separated into two groups: calcium phosphate (apatite or brushite) containing and all others (calcium oxalate monohydrate, calcium oxalate dihydrate, uric acid). We then looked for differences between the phosphate and non-phosphate stones. Significance was determined using Fisher's exact tests for categorical variables and Wilcoxon rank sum tests for continuous variables. RESULTS: 192 renal units underwent PCNL. 144(75%) were metabolic and of these, 73(51%) were phosphate-containing calculi. When comparing phosphate stones to non-phosphate containing stones, females were significantly more likely to have phosphate stones (56% vs. 35%, p< 0.01). Patients with phosphate stones were also more likely to be younger (mean 49 vs. 60 years of age, p<0.001) and have lower BMI's (30 vs 32, p< 0.02). The only stonerelated factor that differed was the average Hounsfield units (HUs) with the phosphate group having brighter stones (1019 vs. 776, p< 0.001). There were no significant differences in stone volume, number of calyces involved, or the S.T.O.N.E nephrolithometry score. Stone free status did not differ. CONCLUSIONS: The majority of large and complex stones undergoing PCNL were metabolic. Of these, 51% had a calcium phosphate component. Patients with phosphate containing stones were more likely to be female, younger and have lower BMI's. These stones were also denser based on HUs. We did not find a decreased stone free rate with phosphate containing stones which has been suggested in other series. Further work is needed to identify the metabolic factors responsible for this shift in stone composition.