title: Parameters of phosphorus homeostasis at normal and reduced GFR: theoretical considerations.
abstract: OBJECTIVE
  Influx and reabsorption of phosphorus (IP and TRP) are assessed with fractional excretion and reabsorption (FEP and FTRP, nl ≤ 20% and ≥ 80%), or with excretion and reabsorption per volume of filtrate (EP/GFR and TRP/GFR, fasting nl ≈ 0.4 and 3.0 mg/dL). We analyzed these parameters at normal and reduced GFR.
  
  
  METHODS
  We equated GFR with creatinine clearance (Ccr) to develop necessary equations. We plotted serum phosphorus ([P]s), EP/Ccr, and FEP against their determinants, and TRP/Ccr against EP/Ccr at FEP of 20% or 40%.
  
  
  RESULTS
  Linear equations related [P]s to EP/Ccr and TRP/Ccr, and EP/Ccr to [cr]s and [P]u/[cr]u (a surrogate for IP). FEP rose in curvilinear fashion as Esub>P/Ccr rose and TRP/Ccr fell; changes in low values of EP/Ccr and TRP/Ccr induced large changes in FEP. At increased EP/Ccr (as in CKD), maintenance of FEP ≤ 20% required impossibly high TRP/Ccr; at EP/Ccr of 2.0 mg/dL, FEP and FTRP of 40% and 60% required normal TRP/Ccr.
  
  
  CONCLUSIONS
  EP/Ccr varies with IP at normal GFR, and with IP and [cr]s at low GFR. FEP, a function of EP/Ccr and TRP/Ccr, varies primarily with the lower ratio, which is always EP/Ccr at normal GFR. At low GFR, high FEP is inevitable if IP is preserved, and TRP/Ccr may be normal despite low FTRP. Contributions of IP and TRP to [P]s should be assessed with EP/Ccr and TRP/Ccr. FEP and FTRP have limitations at any GFR.
authors: Stratton Veterans Affairs Medical Center (MC ), Albany Medical College (MC ), and Albany College of Pharmacy and Health Sciences, Albany, NY, USA.
authors: Kenneth R
authors: KR
authors: Phelps
authors: Darius L
authors: DL
authors: Mason
mesh_terms: D003404:Creatinine / Q000378:metabolism; D005919:Glomerular Filtration Rate; D006706:Homeostasis; D006801:Humans; D007684:Kidney Tubules / Q000378:metabolism; D010758:Phosphorus / Q000378:metabolism*
chemical_list: D010758:Phosphorus; D003404:Creatinine