Low serum calcium is associated with poor renal outcomes in chronic kidney disease stages 3–4 patients

BACKGROUND Mineral disorders are associated with adverse renal outcomes in chronic kidney disease (CKD) patients. Previous studies have associated hypercalcemia and hypocalcemia with mortality; however, the association between serum calcium and renal outcome is not well-described. Whether adding calcium besides phosphorus or in the form of calcium-phosphorus (Ca×P) product into the model of survival analysis could improve the prediction of renal outcomes is not known. METHODS A prospective cohort of 2144 outpatients with CKD stages 3-4 was evaluated. Cox proportional hazard analysis was performed according to calcium quartiles. RESULTS The mean calcium level was 9.2±0.7 mg/dL. Low serum calcium (<9.0 mg/dL) was associated with increased risk of requiring renal replacement therapy (RRT) (hazards ratio [HR]:2.12 (95% CI: 1.49-3.02, P<0.05) and rapid renal function progression (odds ratio [OR]: 1.65 (95% CI: 1.19-2.27, P<0.05) compared with high serum calcium (>9.8 mg/dL). Adding calcium into the survival model increased the integrated discrimination improvement by 0.80% (0.12%-1.91%) while calcium-phosphorus product did not improve risk prediction.The combination of high serum phosphorus (>4.2 mg/dL) and low serum calcium (<9.1 mg/dL) was associated with the highest risk of RRT (HR:2.31 (95% CI: 1.45-3.67, P<0.05). CONCLUSION Low serum calcium is associated with increased risk of RRT and rapid renal function progression in CKD stage 3-4 patients. The integration of serum calcium and phosphorus, but not calcium-phosphorus product should be considered in a predictive model of renal outcome.