Calcium isotope ratios in blood and urine: A new biomarker for the diagnosis of osteoporosis

We assessed the potential of Calcium (Ca) isotope fractionation measurements in blood (δ(44/42)Ca(Blood)) and urine (δ(44/42)Ca(Urine)) as a new biomarker for the diagnosis of osteoporosis. One hundred post-menopausal women aged 50 to 75 years underwent dual-energy X-ray absorptiometry (DXA), the gold standard for determination of bone mineral density. After exclusion of women with kidney failure and vitamin D deficiency (<25 nmol/l) 80 women remained in the study. Of these women 14 fulfilled the standard diagnostic criteria for osteoporosis based on DXA. Both the δ(44/42)Ca(Blood) (p < 0.001) and δ(44/42)Ca(Urine) (p = 0.004) values were significantly different in women with osteoporosis (δ(44/42)Ca(Blood): −0.99 ± 0.10‰, δ (44/42)Ca(Urine): +0.10 ± 0.21‰, (Mean ± one standard deviation (SD), n = 14)) from those without osteoporosis (δ(44/42)Ca(Blood): −0.84 ± 0.14‰, δ(44/42)Ca(Urine): +0.35 ± 0.33‰, (SD), n = 66). This corresponded to the average Ca concentrations in morning spot urine samples ([Ca](Urine)) which were higher (p = 0.041) in those women suffering from osteoporosis ([Ca](Urine-Osteoporosis): 2.58 ± 1.26 mmol/l, (SD), n = 14) than in the control group ([Ca](Urine)-(Control): 1.96 ± 1.39 mmol/l, (SD), n = 66). However, blood Ca concentrations ([Ca](Blood)) were statistically indistinguishable between groups ([Ca](Blood), control: 2.39 ± 0.10 mmol/l (SD), n = 66); osteoporosis group: 2.43 ± 0.10 mmol/l (SD, n = 14) and were also not correlated to their corresponding Ca isotope compositions. The δ(44/42)Ca(Blood) and δ(44/42)Ca(Urine) values correlated significantly (p = 0.004 to p = 0.031) with their corresponding DXA data indicating that both Ca isotope ratios are biomarkers for osteoporosis. Furthermore, Ca isotope ratios were significantly correlated to other clinical parameters ([Ca](Urine), ([Ca](Urine/)Creatinine)) and biomarkers (CRP, CTX/P1NP) associated with bone mineralization and demineralization. From regression analysis it can be shown that the δ(44/42)Ca(Blood) values are the best biomarker for osteoporosis and that no other clinical parameters need to be taken into account in order to improve diagnosis. Cut-off values for discrimination of subjects suffering from osteoporosis were − 0.85‰ and 0.16‰ for δ(44/42)Ca(Blood) and δ(44/42)Ca(Urine), respectively. Corresponding sensitivities were 100% for δ(44/42)Ca(Blood) and ~79% for δ(44/42)Ca(Urine). Apparent specificities were ~55% for δ(44/42)Ca(Blood) and ~71%. The apparent discrepancy in the number of diagnosed cases is reconciled by the different methodological approaches to diagnose osteoporosis. DXA reflects the bone mass density (BMD) of selected bones only (femur and spine) whereas the Ca isotope biomarker reflects bone Ca loss of the whole skeleton. In addition, the close correlation between Ca isotopes and biomarkers of bone demineralization suggest that early changes in bone demineralization are detected by Ca isotope values, long before radiological changes in BMD can manifest on DXA. Further studies are required to independently confirm that Ca isotope measurement provide a sensitive, non-invasive and radiation-free method for the diagnosis of osteoporosis.