Higher residual kidney function in hemodialysis (HD) patients is associated with better survival unless serum phosphorus or intact parathyroid hormone (iPTH) is markedly elevated, a new study finds.

Greater residual kidney function allows better clearance of solutes, removal of uremic toxins, and volume control, and leads to less anemia, inflammation, intradialytic hypotension, erythropoietin resistance, and ventricular hypertrophy, according to researchers. Previous research has linked parameters of chronic kidney disease and mineral and bone disorder (CKD-MBD)—including serum calcium, phosphorus, iPTH, and alkaline phosphatase—to increased risks of mortality.

Mengjing Wang, MD, and Yoshitsugu Obi, MD, of the University of California Irvine Medical Center, and colleagues examined the relationship of CKD-MBD biomarkers with all-cause mortality according to preserved renal function. They modeled data from 35,114 patients who initiated thrice-weekly HD during 2007 to 2011 and stratified results by residual renal function. The investigators used average residual renal urea clearance during the first 91 days of HD and formed 3 groups: less than 1.5, 1.5 to less than 3.0, and 3.0 mL/min/1.73 m2 or higher.

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Over a year of follow up, 23% of patients died, the investigators reported online ahead of print in the Clinical Journal of the American Society of Nephrology. According to Cox proportional hazard models, mortality risk increased along with phosphorus level, particularly among patients with higher residual renal urea clearance. Patients with phosphorus levels at or above 7.0 mg/dL had increased risks of dying regardless of kidney function. Results held after accounting for a range of demographic, clinical, and laboratory variables. Patients’ high phosphorus levels probably indicated poor adherence to diet and medication and high-turnover bone disease, according to the investigators.

In addition, the study found increased mortality among patients with lower iPTH and low residual renal urea clearance or high iPTH and higher residual renal urea clearance, using 1.5 mL/min/1.73 m2 as a cutoff.

Patients with elevated serum corrected total calcium or alkaline phosphatase concentrations had greater risks of dying despite their residual kidney function.

Overall, greater preserved kidney function was associated with better survival. The findings support another recent study by the investigators highlighting the importance of residual kidney function in the HD population.

Clinical applications of the research have yet to be determined. “Future studies with a long-term follow-up period are needed to explain the underlying mechanisms of those associations and examine whether taking account for residual kidney function in the assessment of mortality risk associated with serum phosphorus and intact PTH improves patient management and clinical outcomes among patients on hemodialysis,” the investigators concluded.

Among the study limitations, the team could not investigate some relevant confounders, such as vitamin D deficiency, inflammatory status, elevated fibroblast growth factor-23, treatment adherence, and poor predialysis care.

In addition, renal urea clearance may not be the most accurate measure of residual kidney function. Once a reliable method of estimating renal kidney function is found, it might ultimately improve standard of care for HD patients.

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Wang M, Obi Y, Streja E, et al. Association of parameters of mineral bone disorder with mortality in patients on hemodialysis according to level of residual kidney function. Clin J Am Soc Nephrol 12. doi: 10.2215/CJN.11931116.