H4i: Describe the principles of measurement of GFR

Glomerular Filtration Rate

GFR = the volume of plasma filtered by the kidneys per minute → normally 125mL/min

GFR Measurement

  • Properties of an indicator substance
  1. Stable → not metabolised/degraded/physiologically reactive
  2. Non-toxic
  3. Easy to measure
  • Use of clearance to measure GFR
    • RENAL CLEARANCE = the volume of plasma completely cleared of a substance by the kidney per unit of time
Clearance1

(CLEARANCE = UV/P)

  • So renal clearance can be used to measure GFR if a substance measured only reflects filtration
  • ∴it will have to:
    1. Freely filtered at renal corpuscles
    2. Not reabsorbed
    3. Not secreted
    4. Not synthesised by tubules

Inulin = a polysaccharide that fits above description

  • Problem with inulin:
    • Not naturally occurring in body
    • Delivered by an IV infusion until reaches steady state
    • You need a urine collection over a fixed time whilst it is at SS

Creatinine

  • It is a lesser substitute
  • Formed from muscle creatine
  • Released into blood at constant rate ∴not fluctuating over 24hrs
  • Of the criteria, it does not meet 1 because it is secreted by tubules
    • ∴provides small OVERESTIMATE of GFR
  • Creatinine varies with muscle bulk
    • ∴corrected for age, weight, sex by COCKROFT-GAULT equation
  • There is also a non-linear relationship between serum creatinine & creatinine clearance
  • Renal function ↓50% before a significant ↑CrT plasma is seen