QS Study

The quantity of glomerular filtrate formed per minute in all the functioning nephrons of both kidneys is called glomerular filtration rate (GFR). It is a test used to check how well the kidneys are working. Specifically, it estimates how much blood passes through the glomeruli each minute. A GFR of 60 or higher is in the normal range. A GFR below 60 may mean kidney disease. A GFR of 15 or lower may mean kidney failure.

Measurement of GFR

Principle

GFR can be measured by measuring urinary and plasma level of a substance that is freely filtrated through glomeruli and neither secreted nor reabsorbed by tubules.

Substances used for measuring GFR –

  • Inulin
  • Mannitol
  • Sorbitol
  • Sucrose
  • EDTA
  • Radioactive cobalt

Method

  • A loading dose of Inulin is administrated intravenously followed by a sustaining infusion.
  • After the inulin has mixed with body fluid equally, then urine sample is collected.
  • A plasma sample is collected, halfway through the collection.
  • Urine and plasma conc of Inulin is determined and clearance is calculated.

Calculation

The GFR is equal to the conc of Inulin in urine (Un) times the urine flow (Vn) per unit of time divided by the plasma level of Inulin (Pn). This is called clearance of Inulin (Cin). Clearance of Inulin is equal to GFR.

Example

Un = 0.125 g/ml; Vn = 1 ml/min

n = Insulin; Pn = 0.001 g / ml

GFR or Cin =  (Un x Vn) / Pn

= (0.125 x 1) / 0.001 = 125 ml/min

  1. It is not reaborbed and passes through glomerular membrane as freely as crystalloid
  2. Non-toxic and biologically inert
  3. Not metabolized by the body
  4. It is not actively secreted by the tubules
  5. It does not bind to plasma proton.
  6. It is not stored in kidney and produces no pathogenesis.
  7. It has no effect on filtration rate
  8. Easily measure both in plasma and in urine.
  9. Available and less cost.
  10. It has no physiological influence.