The 5-year reproducibility of calcium-related biochemical variables in postmenopausal women

A total of 19 measured and derived bone-related biochemical variables were determined in 307 postmenopausal volunteers on two occasions, 5 years apart. The plasma variables with the highest coefficients of determination (r2) were plasma globulins, alkaline phosphatase, creatinine and calculated ionized and ultrafiltrable calcium. In the urine, the highest r2 values were in respect of fasting urine calcium excretion corrected for urine sodium, hydroxyproline excretion, and the maximal renal tubular reabsorption of calcium and phosphate (TmCa/GFR and TmP/GFR). The components of variance of TmCa/GFR and TmP/GFR show marked individuality but their methods determination meet the criterion for acceptable analytical goals. We conclude that most of the measured and derived bone-related biochemical variables in fasting plasma and urine are sufficiently reproducible in postmenopausal women to be useful for ranking individuals for a period up to 5 years.