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

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