NDT Advance Access originally published online on February 11, 2009
Nephrology Dialysis Transplantation 2009 24(7):2123-2131; doi:10.1093/ndt/gfp040
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Glomerular filtration rate and serum phosphate: an inverse relationship diluted by age
1 Nephrology, Second University of Naples, Italy 2 Centre for Preventive Medicine, Gubbio 3 Merck, Italy
Correspondence and offprint requests to: Massimo Cirillo; E-mail: massimo.cirillo{at}unina2.it
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Abstract |
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Background. Available data indicate that serum phosphate increases only when glomerular filtration rate (GFR) falls into the low range (<60 mL/min x 1.73 m2). GFR and serum phosphate decrease with ageing. This population-based study investigated by age-controlled analyses the relationship of GFR with serum phosphate in adults with GFR above the low range.
Methods. Data were collected on age, sex, menstrual status, anthropometry, overnight urinary creatinine, dietary protein (overnight urinary urea), reported intake of milk/yogurt, serum creatinine, phosphate, calcium and total protein in 4034 adults (age 18–91 years) with GFR 
60 mL/min x 1.73 m2 as assessed by estimated GFR (eGFR, simplified MDRD equation) and creatinine clearance (overnight urinary creatinine/serum creatinine).
Results. The relationship of eGFR with serum phosphate was positive in men and null in women in univariate analyses (P = 0.001 and 0.148), negative in both sexes with age adjustment (P < 0.001). Age-adjusted results did not depend on colinearity between age and eGFR because the relationship was inverse also replacing eGFR with creatinine clearance (P < 0.001 in both sexes). In univariate regression analysis done separately by gender and six age-strata (18–24, 25–34, 35–44, 45–54, 55–64 and 65), the line of serum phosphate over eGFR was constantly inverse (range of P = 0.010/0.089) with the progressively lower y-axis intercept from young to older ages. The inverse relationship of eGFR or creatinine clearance with serum phosphate was significantly inverse also controlling for other variables (P < 0.01).
Conclusions. GFR differences in the range 60 mL/min x 1.73 m2 are inversely and independently related to serum phosphate. The relationship is undetectable without age-controlled procedures because, for serum phosphate, the effect of GFR differences above 60 mL/min x 1.73 m2 is much smaller than the effect of age.
Keywords: age; GFR; Gubbio study; phosphate
Received for publication: 17. 9.08
Accepted in revised form: 20. 1.09