Measuring Kidney Function

Question 1

Define clearance

Answer 1

Clearance is the volume of plasma cleared of a substance per unit of time

Question 2

How do you calculate clearance

Answer 2

• Renal clearance of a substrate = excretion rate / plasma concentration
• Excretion rate = U (amount in urine) x V (urine flow rate)

Question 3

Define GFR and state normal values

Answer 3

• GFR is the amount of filtrate that is produced from the blood flow per unit of time
• Normal GFR is 90-120 mL/min/1.73m2
• Normal total glomerular filtrate per day is 140-180 L/day
• Lower boundary is girls, upper boundary is boys

Question 4

Explain the ideal parameters for a substance to measure kidney clearance

Answer 4

• Be produced at a constant rate
• Be freely filtered across glomerulus
• Not be reabsorbed in the nephron
• Not be secreted into the nephron
• Then excretion rate = GFR

Question 5

Explain the use of inulin in measuring clearance

Answer 5

• Inulin is an exogenous substance that is injected and a marker of GPR
• However inulin requires continuous monitoring and connected to machine to maintain steady state
• Requires catheter and timed urine collections

Question 6

Explain the use of 51 Cr-EDTA in measuring clearance

Answer 6

• Radioactive labelled marker cleared exclusively by renal function
• Timed injection with blood samples taken 2,3,4 hours afterward - don’t need catheter but needs 3 blood samples
• Approximately 10% lower clearance than inulin - account for this in calculation
• Used clinically in children
  
  • Used where indication of renal function required (kidney transplant or work up to donate kidney for transplant)

Question 7

Explain the use of creatinine in measuring clearance

Answer 7

• Creatinine clearance is an overestimate of GFR
  
  • GFR increased as secreted into nephron
• Creatinine clearance measured by collecting urine over 24 hours and measuring serum creatinine
• Used in pregnancy
• Creatinine levels increased through muscle cell breakdown, large muscle bulk, male, black, creatine supplements, meat
  
  • Decreased through reduced muscle mass, old, Hispanic/Indo-Asian, female, vegetarian

Question 8

Explain the concept of eGFR

Answer 8

• Estimate glomerular filtrate rate taken from serum creatinine levels to account for differences between individuals
• Equation used based on serum creatinine, age, sex and Caucasian or black

Question 9

Explain the drawbacks of eGFR

Answer 9

• MDRD eGFR underestimates true GFR when serum creatinine close or within normal range
  
  • Risk of people being labelled as having chronic kidney disease
• Inaccurate in:
  
  • People without kidney disease (transplant donors)
  • Children
  • Pregnancy
  • Old age
  • Other ethnicities
  • Amputees - significant reduced muscle mass
  • Patients with higher levels of kidney function
    
    • When true GFR changes quickly (acute kidney injury)