How do calcium phosphate stone formers differ? clinical correlates by stone subtype

INTRODUCTION: Calcium phosphate (CaP) stone formers can be subtyped into hydroxyl forms(CaPH-hydroxyapatite or brushite) or carbonate forms (CaPC).Despite increasing CaP stone prevalence,little has been reported on clinical correlates of these stone subtypes. METHODS: We conducted a cross-sectional analysis of all patients referred to our institution for endourologic procedures between 2009-12 (n=288).Inclusion criteria included stone analysis containing more than 50% CaPH or CaPC and 3 month follow-up 24 hour urine collection.Stone type was analyzed against patient demographics and clinical parameters,BMI,comorbidities,recurrent urinary tract infection,positive cultures,and number of previous stone episodes and previous stone-related procedures. ANOVA and chisquare test were used to compare means and proportions,respectively, between CaP subtypes. RESULTS: Twenty patients with >50% CaPH and 15 patients with >50% CaPC stones met inclusion criteria.Both groups were similar in regard to procedural age (45±15 years), previous stone passages (3±3) and stone procedures (2±0.7).CaPC patients were more likely to be diagnosed with morbid obesity (p=0.02).14/15(93%) of CaPC patients had history of culture-positive UTI (predominately urease splitters) compared to 12/20(60%,p<0.02) CaPH patients (predominantly E. coli).For associated mineral types,10/15(67%) CaPC patients vs. 0/20(0%) of CaPH patients had associated struvite stones (p<0.001).CaPC patients had lower 24 hour urine citrate (283±161 mg/day vs.494±270 mg/day,p<0.02) and higher pH (6.20±0.07 vs. 6.07±0.05,p<0.03) than CaPH stone formers. CONCLUSIONS: CaPC stones are highly associated with morbid obesity, recurrent UTI, struvite mineral stone type, hypocitrituria, and elevated urine pH when compared to CaPH form. Future studies should focus on the role of infection eradiction, weight loss, and complete stone removal on long-term outcomes of CaPC stone formation.