Validation of a simple isotope method for estimating true calcium fractional absorption in adolescents

Summary We validated a single oral isotope method for estimating fractional calcium absorption determined by double isotope methods in adolescents. Developed equations with an oral isotope including a single blood draw or spot urine collection can be used to evaluate fractional calcium absorption in adolescents which allows flexibility in developing protocols. Introduction This study was designed to develop and validate a simpler, less expensive single oral isotope method for determining fractional calcium (Ca) absorption in adolescents. Methods We used our database of 31 observations from ten white and 12 black adolescent girls aged 10-15 years who participated in metabolic and kinetic studies. Tracer data following oral (⁴⁴Ca) and intravenous (IV, ⁴²Ca) administration of calcium stable isotopes and samples in serum and urine from various time points up to 4 days were used to develop methods using multiple regression analysis based on a single measurement of enriched stable isotope/tracee defined as tracer/tracee (TT) in serum (TTserum) or urine (TTurine). Reference values for fractional calcium absorption were from oral/IV stable isotope ratios in 24-h serum or urine and full kinetic modeling. Results The strongest equation using a single blood sample had R ² = 0.94 (p < 0.001): fractional Ca absorption = 1.3340(4-h TTserum)⁰.⁷⁸⁷² BSA¹.⁷¹³²e ⁽⁻⁰.⁰¹⁶⁵² PMA⁾, where BSA is body surface area and PMA is post-menarcheal age. The strongest equation using a single urine sample had R ² = 0.95 (p < 0.001): fractional Ca absorption = 2.3088 (5-12 h TTurine)⁰.⁸²⁰⁸ BSA¹.⁵²⁶⁰e ⁽⁻⁰.⁰¹⁸⁵⁰ PMA⁾. Equations were also developed with Tanner score. An external data set of Asian adolescent boys and girls was used to validate the equations. Conclusion Equations using an oral isotope and a single blood draw or urine collection for determining fractional calcium absorption were successfully validated in healthy, non-obese white and black adolescent girls aged 10-15 years. The equations well-predicted fractional calcium absorption in Asian adolescent boys and girls.