The Calcium/Phosphorus Homeostasis in Chronic Kidney Disease: From Clinical Epidemiology to Pathophysiology

Introduction: A simple data filtering process together with some basic concepts of control theory applied to electronically stored clinical data were used to identify some of the pathophysiological mechanisms underlying the perturbations of the calcium/phosphorus homeostasis in chronic kidney disease. Material and Methods: Retrospective data (a set per patient of serum single value concentrations of creatinine, calcium, phosphorus, parathormone, 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D) from 2507 patients with stable chronic kidney disease not on renal replacement therapy were studied. The variables were paired and subjected sequentially to a moving average and partioned into frequency classes. The plots were interpreted using the concept of a feedback loop comprising two branches of opposite sign and of set point of the loop. The set point for each pair of variables is displaced in the course of the disease and this displacement indicates which of the two factors involved (the serum concentrations of calcium or parathormone, for example) is primarily affected. Results: This analysis showed that in the course of the development of chronic kidney disease the relationships between the observed variables progressed following a monotonous, a biphasic or a triphasic pattern. Discussion: As chronic kidney disease progresses, calcium/phosphorus metabolism regulation evolves through different phases. Later, there is a progressive loss of the parathyroid gland sensitivity to the control by the serum concentrations of calcium and phosphorus. The sensitivity to the inhibitory action of 1,25-dihydroxyvitamin D decreases monotonously but never releases the gland. Conclusion: The clinical data analysis used permits to illustrate the underlying pathophysiological mechanisms.