Serum phosphorus and calcium levels, and kidney disease progression in immunoglobulin A nephropathy

Background. Disorders of calcium and phosphorus metabolism have been reported to be associated with all-cause and cardiovascular mortality in patients requiring long-term dialysis therapy. However, its role in disease progression is not well established in patients without dialysis, especially in immunoglobulin A (IgA) nephropathy. We aim to evaluate the association of serum phosphorus and calcium and progression of IgA nephropathy. Methods. We assessed 2567 patients with IgA nephropathy at the First Affiliated Hospital, College of Medicine, Zhejiang University. Serum phosphorus and calcium were collected at the time of kidney biopsy and at each visit. The associations of serum phosphorus and serum calcium with composite kidney disease progression events, defined as 50% estimated glomerular filtration rate (eGFR) decline and kidney failure, were examined using Cox models and restricted cubic splines. Results. During a median follow-up of 31.9 months, 248 (10%) patients reached composite kidney disease progression events. A linear relationship was observed between serum phosphorus and composite kidney disease progression events. With higher levels of phosphorus, the risk of kidney disease progression events increased {hazard ratio [HR] 3.54 [95% confidence interval (CI) 1.37-9.12]; P=0.009}. Compared with the first quartile group, the HR of kidney disease progression events was 1.66 (95% CI 0.91-301) for the second quartile, 1.67 (95% CI 0.91-3.08) for the third and 2.62 (95% CI 1.44-4.77) for the fourth (P for trend = 0.002). The association between serum phosphorus and kidney disease progression was detectable [HR 8.94 (95% CI 2.33-34.21); P=0.001] within the subgroup with eGFR <60 mL/min/1.73 m(2) but not among patients with eGFR >= 60 mL/min/1.73 m(2) [HR 0.87 (95% CI 0.17-4.44); P=0.87]. After adjustment for traditional risk factors, a higher level of serum calcium was not associated with kidney disease progression events [HR 0.33 (95% CI 0.10-1.09)]. Conclusions. Higher serum phosphorus rather than serum calcium was independently associated with kidney disease progression in IgA nephropathy.