Reference range for 24-h urine calcium, calcium/creatinine ratio, and correlations with calcium absorption and serum vitamin D metabolites in normal women

Summary: A study of 959 black and white women shows that the normal range for 24-h urine calcium excretion in white women is 30–300 mg and 10–285 mg in black women. Clinical laboratories use a normal range for 24-h urine calcium excretion of 100–300 mg; there is a need for them to update their age- and race-specific ranges. Introduction: Recommendations for a normal range for 24-h urine calcium vary from a low of 50 mg to a high of 400 mg; most “laboratory normal ranges” based on older literature are incorrect. The objective of this analysis is to define a normal range for young women age 25–45 years and older women age 55–90 years, white and black, for 24-h urine calcium and calcium/creatinine ratio, and to examine the relationship between 24-h urine calcium, calcium absorption, and vitamin D metabolites. Methods: Data from 3 studies was collected on 959 normal black and white women, ages 25–87 years, for 24-h urine calcium, creatinine, calcium intake, serum 25-hydroxyvitamin D (25OHD),1,25 dihydroxyvitamin D (1,25(OH)₂D), and calcium absorption. Urine calcium and creatinine were measured on an auto-analyzer. Calcium absorption was measured by a single isotope method. Serum 25OHD and 1,25(OH)₂D were measured by competitive protein binding assay or radioimmunoassay. Because 24-h urine calcium did not fit a normal distribution, non-parametric methods were used to determine the 95% reference interval (RI) and 90% non-parametric confidence intervals (CI) calculated for groups. Results: The 95% reference intervals for 24-h urine calcium for women were as follows: black older 7–225 mg, black younger 8–285 mg; white older 37–275 mg, and white younger 23–287 mg. In older white women, 24-h urine calcium was significantly correlated with serum 1,25(OH)₂ D, r = 0.23, p < 0.001, calcium intake r = 0.12, p = 0.001, and calcium absorption r = 0.18, p = 0.003, but not serum 25OHD r = 0.07, p = 0.06. Conclusions: The normal reference interval for 24-h urine calcium for black women is lower than white women. Twenty-four-hour urine calcium was correlated with serum 1,25(OH)₂ D calcium intake and calcium absorption, but not serum 25OHD. This range will be useful clinically for defining hypercalciuria and for following patients on vitamin D and calcium treatment.