• volume of plasma filtered per unit time
• best index of functioning renal mass
• Serial measurements are used clinically to assess severity and course of renal disease.
• ß GFR = ß renal function (ie, disease)
• Ý GFR = improvement in renal function
• stable but subnormal GFR = stable renal disease
• Normal value for total kidney GFR =125mL/min men = 115-130 mL/min women = 100-125 mL/min
• Necessary for determining drug dose: Patient with renal disease -normal [drug]_plasma can accumulate to toxic levels

• The volume of plasma cleared of a substance by renal excretion
• always in units of Volume/time: usually mL/min
• Inulin – used to estimate GFR (glomerular filtration rate)
• clearance of inulin accurately estimates GFR because (like all GFR markers) it:
• achieves stable plasma concentration
• is freely filtered at the glomerulus
• is not reabsorbed, secreted, synthesized or metabolized by kidney
• Therefore the amount of inulin found in urine per unit time reflects the amount of inulin filtered from plasma per unit time. (Filtered inulin = Excreted inulin)
• If plasma inulin concentration is known, then the volume of plasma filtered per unit time (GFR) can be calculated as follows: GFR = urine inulin concentration X urine volume over time / plasma inulin concentration
• C_inulin (GFR) is measured by infusing inulin and measuring plasma and urine inulin levels and urine volume per unit time
• Para-aminohippurate – used to estimate RPF (renal plasma flow)
• Clearance of PAH (para-aminohippurate) accurately estimates renal plasma flow (RPF) because:
• It is freely filtered from the glomerulus
• is completely secreted from the pertitubular capillaries to the proximal tubules
• C_x = [X]_urine x V/[X]_plasma X = any molecule
• Clearance of any molecule relative to inulin describes renal handling of that compound
• C_x = C_inulin no net reabsorbtion or secretion of x
• C_x < C_inulin net reabsorption of x (ie, urea)
• C_x > C_inulin net secretion of x

Creatinine Clearance: Common clinical estimate of GFR: Creatinine is an endogenouse marker (less cumbersome)

• C_creatinine determined using venous blood creatinine concentration and 24 hr urine collection for V and urine creatinine concentration
• Creatinine derives from creatine metabolism in the skeletal muscle (relatively constant production)
• Is basically not reabsorbed or synthesized at the kidney
• C_cr is only an estimate of GFR: several factors introduce error (ie, small amount of Cr is secreted by tubules)
• Has stable plasma levels: Cr_excretion= Cr_production at Steady State (SS)
• Is freely filtered at the glomerulus: GFR x [Creatinine]_plasma = Constant; therefore ß GFR Þ Ý [Cr]_Plasma until new SS
• The inverse relationship between GFR and [Cr]_plasma is a somewhat insensitive indicator of GFR
• 1.4mg/dl is considered a normal [Cr]_plasma level. However, due to Ý tubular secretion and hyper filtration of creatinine in renal disease many patients present with [Cr]_plasma near this range inspite of greatly reduced GFRs (40-80ml/min).
• Creatinine secretion becomes saturated at [Cr]_plasma above 1.5-2 mg/dl. therefore [Cr]_plasma rise as expected with further decreases in GFR
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