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Can 24-hour urine values differentiate between patients forming calcium phosphate (CaP) and calcium oxalate (CaOx) stones?
Troy Larson, MD, Lisa Wang, MD, Virginia Hood, MD, Peter Callas, PhD, Kevan Sternberg, MD
University of Vermont, Burlington, VT, USA.

Background: Stone composition is often unknown or reported as "calcium based" by patients being evaluated for recurrent nephrolithiasis. Distinguishing between CaP and CaOx stones is clinically important as dietary and pharmacological preventative strategies differ. We investigated whether 24-h urine values and patient factors can be used to differentiate between patients forming CaP and CaOx stones.
Methods: A retrospective review was performed of patients who had 24-h urine values (Litholink, Chicago, Il) and associated stone analysis data from a single academic medical center. Patients whose stone composition was predominantly (>50%) CaP or predominantly (>50%) CaOx were identified. In those with urine studies within 3 months of the stone analysis, patient characteristics and 24-h urine values were compared between the groups forming CaP and CaOx stones using t tests for continuous variables and chi square tests for categorical variables. Multivariate logistic regression was used to determine which variables were independently associated with stone type.
Results: From 2006 to 2014, 2019 urine values were obtained from 1197 patients. Of these, 371 had a calcium based stone identified within 3 months of the urine collection (Table). 65 (18%) had a predominantly CaP stone; 44 (68%) of these were female. 306 (82%) had a predominantly CaOx stone; 121 (40%) of these were female. With multivariate logistic regression, CaP stones were more likely to occur in females (OR 2.25, 95% CI 1.19-4.26), with decreasing age (OR per 10 year decrease in age = 1.42, 95% CI 1.15-1.75), with decreasing urine citrate (OR per 100 unit decrease = 1.20, 95% CI 1.06-1.35), with increasing pH (OR per 1 unit increase = 3.24, 95% CI 1.71-6.13), or increasing Ca24/Cr24 (OR per 50 unit increase = 1.26, 95% CI 1.03-1.55). No differences were found between the stone types for BMI, 24-h urine volume or phosphate.
CaP (n=65)CaOx (n=306)
meanSDmeanSDp -value
Ageyears45135414<0.0001
pH6.360.496.030.51<0.0001
Oxmg/day321038150.0003
Citmg/day4952876163030.003
SSCaP1.791.101.250.98<0.0001
SSUA0.630.810.980.900.004
Crmg/day149244216394340.01

Table - Univariate analysis of 24-h urine values between CaP and CaOx stone formers.
Conclusion: Female sex, decreasing age, increasing urine pH, and decreasing urine citrate were associated with predominantly CaP stones. This knowledge can be used to help differentiate between patients forming CaP and CaOx stones, may give insight into the etiology of CaP stone formation, and may help guide treatment for those whose stone composition is not known.


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