Calcium isotopes as a biomarker for vascular calcification in chronic kidney disease

Abstract

Calcium balance is abnormal in adults with chronic kidney disease (CKD) and is associated with the development of vascular calcification. It is currently not routine to screen for vascular calcification in CKD patients. In this cross-sectional study, we investigate whether the ratio of naturally occurring calcium (Ca) isotopes, 44Ca and 42Ca, in serum could be used as a noninvasive marker of vascular calcification in CKD. We recruited 78 participants from a tertiary hospital renal center: 28 controls, 9 subjects with mild-moderate CKD, 22 undertaking dialysis and 19 who received a kidney transplant. For each participant, systolic blood pressure, ankle brachial index, pulse wave velocity, and estimated glomerular filtration rate were measured, along with serum markers. Calcium concentrations and isotope ratios were measured in urine and serum. While we found no significant association between urine Ca isotope composition (noted δ44/42Ca) between the different groups, δ44/42Ca values in serum were significantly different between healthy controls, subjects with mild-moderate CKD and those undertaking dialysis (P < 0.01). Receiver operative characteristic curve analysis shows that the diagnostic utility of serum δ44/42Ca for detecting medial artery calcification is very good (AUC = 0.818, sensitivity 81.8% and specificity 77.3%, P < 0.01), and performs better than existing biomarkers. Although our results will need to be verified in prospective studies across different institutions, serum δ44/42Ca has the potential to be used as an early screening test for vascular calcification.

Keywords: biomarker; calcium; chronic kidney disease; isotopes; mineral balance; vascular calcification.

Graphical Abstract

The calcium isotope composition (noted δ^44/42Ca) of blood serum of adults with chronic kidney disease can be used as a screening tool to detect medial vascular calcification, where a δ^44/42Ca value greater than −0.53‰ indicates a high risk of medial vascular calcification.

Fig. 1

Boxplot of urine calcium concentrations (determined by ICP–MS) across the four groups. All figures were drawn using R package ggplot2.

Fig. 2

Urine Ca concentration (in μg/g) as a function of FGF23. The curve shows a local polynomial regression and the gray area represents the confidence interval at 0.9 level on the regression. Regressions and their confidence intervals were drawn in this and following figures using R package ggplot2 with a span value (degree of smoothing) of 0.75 for polynomial regressions.

Fig. 3

Boxplot of serum δ^44/42Ca values across the four groups.

Fig. 4

Serum δ^44/42Ca (in ‰) as a function of dietary Ca intake (in mg), for three groups where both data were available. The lack of relationship suggests that dietary Ca may not have a major influence of serum Ca isotopes in our cohort.

Fig. 5

Serum δ^44/42Ca (in ‰) as a function of whether calcium supplement/medication was taken. On the x-axis, “0” indicates no Ca supplement/medication taken, while “1” indicates Ca supplement/medication was taken. Note there was no serum δ^44/42Ca data for the control subjects taking Ca supplement (N = 2), nor for the mild–moderate CKD subjects not taking Ca supplement (N = 3). The similarity in Ca isotope values for each transplant or dialysis, whether Ca supplement/medication was taken or not, suggests that Ca supplement/medication does not have a major influence on serum δ^44/42Ca values.

Fig. 6

Serum Ca isotope composition as a function of (a) log-transformed FGF23; (b) log-transformed creatinine; (c) pulse wave velocity; and (d) vitamin D (1,25). The lines show linear regressions and gray areas represent the confidence interval at 0.9 level.

Fig. 7

ROC curve for Ca isotopes in serum as medial calcification predictor.