Calcium isotope ratio (δ44/42Ca) reflects kidney function and calcium excretion but not bone mineral density or bone turnover: a cross-sectional study

Calcium isotope fractionation (δ⁴⁴ / ⁴²Ca) was evaluated as a biomarker of bone metabolism in 301 patients. δ⁴⁴ / ⁴²Ca correlated with urinary calcium excretion but not with bone mineral density or fracture status. Age, kidney function, and calcium intake influenced results, limiting its diagnostic utility for osteoporosis in clinical practice. Data on Calcium isotope fractions (δ 44/42 Ca) in isolated bone diseases are limited. Previously proposed as a diagnostic marker in metabolic bone disorders, we now extend our findings from monogenic bone diseases to patients with osteoporosis and osteopenia. In 301 patients (43 controls), serum and urine δ 44/42 Ca were analyzed alongside bone (such as bone density) and treatment parameters. Age, kidney function, and calcium intake significantly affected δ 44/42 Ca and must be considered in interpretation. δ 44/42 Ca correlated strongly with urinary calcium excretion (serum: r²=0.55, urine: r²=0.20; p