Serum Phosphate and Mortality in Patients with Chronic Kidney Disease

Background and objectives: Higher phosphate is associated with mortality in dialysis patients but few prospective studies assess this in nondialysis patients managed in an outpatient nephrology clinic. This prospective longitudinal study examined whether phosphate level was associated with death in a referred population.Design, setting, participants & measurements: Patients (1203) of nondialysis chronic kidney disease (CKD) in the Chronic Renal Insufficiency Standards Implementation Study were assessed. Survival analyses were performed for quartiles of baseline phosphate relative to GFR, 12-month time-averaged phosphate, and baseline phosphate according to published phosphate targets.Results: Mean (SD) eGFR was 32 (15) ml/min per 1.73 m², age 64 (14) years, and phosphate 1.2 (0.30) mmol/L. Cox multivariate adjusted regression in CKD stages 3 to 4 patients showed an increased risk of all-cause and cardiovascular mortality in the highest quartile compared with that in the lowest quartile of phosphate. No association was found in CKD stage 5 patients. Patients who had values above recommended targets for phosphate control had increased risk of all-cause and cardiovascular death compared with patients below target. The highest quartile compared with the lowest quartile of 12-month time-averaged phosphate was associated with an increased risk of mortality.Conclusions: In CKD stages 3 to 4 patients, higher phosphate was associated with a stepwise increase in mortality. As phosphate levels below published targets (as opposed to within them) are associated with better survival, guidelines for phosphate in nondialysis CKD patients should be re-examined. Intervention trials are required to determine whether lowering phosphate will improve survival.Higher serum phosphate is associated with mortality in hemodialysis patients (1–3). There have been three studies in patients with chronic kidney disease (CKD), not on dialysis, evaluating the association of serum phosphate with mortality; two of these found a positive association (4–6). The relationship between serum phosphate and mortality in patients with CKD stages 3 to 5 (eGFR <60 ml/min per 1.73 m²) who are not on dialysis, and who are under regular nephrological review, has not previously been examined in a prospective systematic way. Survival data according to follow-up phosphate results are also lacking in CKD patients.The aims of this single-center study were to investigate whether an association of serum phosphate with all-cause and cardiovascular mortality could be shown prospectively in outpatients with advanced CKD (stage 5) not receiving dialysis and also in those with earlier (stages 3 and 4) CKD. The relationship of 12-month time-averaged phosphate and mortality and the influence upon survival of a serum phosphate within guideline targets was examined in this population.