Supplementary Material for: Relationship between Anemia and Serum Concentrations of Calcium and Phosphorus in Advanced Non-Dialysis-Dependent Chronic Kidney Disease

Background/Aims: Different biochemical abnormalities of metabolic bone disease have been associated with anemia of chronic kidney disease (CKD), mainly in hemodialysis patients. However, all of these abnormalities are closely inter-related and their individual effect on the development of anemia is uncertain. This study was aimed to assess the relationship between anemia and a set of metabolic bone disease biomarkers in a cohort of adult patients with advanced non-dialysis-dependent CKD. Methods: The sample consisted of 382 patients submitted to a Nephrology Unit for evaluation of advanced CKD in a tertiary hospital from Gran Canaria during 3 years. Associations between anemia and serum levels of calcium (albumin-corrected), phosphorus, PTH, 25-hydroxivitamin D (25(OH)D₃) and alkaline phosphatase were analyzed by using logistic regression models with adjustment for other demographic, clinical and biochemical covariates potentially related to anemia and to bone mineral metabolism. Results: Serum levels of calcium and 25(OH)D₃ (negatively) and phosphorus (positively) were significantly associated with anemia in an unadjusted logistic regression model. In a fully adjusted multivariable model, the OR for anemia was 0.29 (95% CI 0.16-0.49; p < 0.0001) for every 1 mg/dl increase in serum calcium and 2.19 (95% CI 1.55-3.15; p < 0.001) for every 1 mg/dl increase in serum phosphorus. Female sex and lower serum albumin levels were also independently associated with anemia. The inclusion of albumin in the adjusted model displaced the significance of 25(OH)D₃. Conclusions: Circulating levels of calcium and phosphorus are strongly linked to anemia in patients with advanced non-dialysis CKD.