6 Measurement of Kidney Function, GFR and Clearance

Question 1

List the factors which may determine the GFR of an individual. (eg age)

Answer 1

• Age
• Gender
• Size of individual
• Size of kidneys
• Pregnancy

Question 2

What is the normal GFR range?

Answer 2

90-120 mL/min/1.73m²

Question 3

Nephron development has finished by which week of foetal development?

Answer 3

35th-36th week

(premature babies have lower nephron numbers)

Question 4

At birth a baby is born with a GFR of about 20mLs/min/1.73m².

By what age should their GFR be normla (90-120mLs/min/1.73m²)?

Answer 4

18 months

Question 5

At what age does GFR start to decline? Why does it decline and what happens as a result of this?

Answer 5

30yrs

Loss of functioning nephrons

Compensatory hypertrophy

Question 6

What happens to GFR during pregnancy?

Answer 6

Increase by about 50%

(130-180 mL/minute)

Kidney size increases about 1cm (but nephron number same)

Back to pre-preganancy levels 6 months post-partum

Question 7

What does a decrease in GFR indicate?

Answer 7

Diseases kidney

Decline in number of nephrons/GFR within individual nephrons

Question 8

Why might GFR not fall significantly until significant kidney damage has occured?

Answer 8

Individual nephrons may hypertrophy

Kidney function declines slowly

Question 9

What is clearance?

Answer 9

Clearance= volume of plasma cleared of a substance (x) per unit of time

(from the whole body not just kidneys)

Question 10

What is renal clearance (what’s the equation)?

Answer 10

Question 11

What must a substance used to measure kidney clearance be? (4)

(ie what the the postulates for the substance measuring GFR)

Answer 11

1. Produced at constant rate
2. Not reabsorbed in nephron
3. Not secreted in nephron
4. Be freely filtered across glomerulus

Question 12

What is inulin?

Answer 12

Plant polysaccharide ingested into body

Question 13

Name some exogenous markers which can be used to measure eGFR?

(eGFR= estimated GFR)

Answer 13

Inulin

51 Cr-EDTA

Iohexol

Question 14

Why is 51 Cr-EDTA (radio-active labelled marker) a better to use to measure kidney function than Inulin?

Answer 14

• Inulin
  
  • requires continuous IV to maintain steady state
  • requires catheter and timed urine collection
• 51 Cr-EDTA
  
  • Single, timed injection
  • Blood samples taken 2,3,4 hrs after
  • Cleared exclusively by renal filtration
    
    • Used in children and to find out kidney function of donors

BUT 51 Cr-EDTA- has 10% lower clearance than inulin

Question 15

What endogenous substance do we use to measure GFR?

Answer 15

Creatinine

Question 16

How do we measure creatinine?

(normal serum creatinine= 70-150 µmol/L)

Answer 16

• Urine creatinine over 24hrs
• Serum creatinine

Question 17

What are the disadvantages of using creatinine as a measure for GFR?

Answer 17

• Not being produced at constant rate= end product of muscle breakdown
• Overestimates GFR 10-20%
  
  • 10% of it is secreted into the nephron
• Other factors affecting creatinine secretion
  
  • small amount secreted by gut
• Less accurate than inulin

BUT:

• Serum creatinine stable in an individual in steady state
• Not reabsorbed
• Freely filtered at glomerulus

Question 18

Name 2 models that are used to estimate GFR from serum creatine levels.

Answer 18

• MDRD eGFR
  
  • ​4 variables: Serum creatinine, age, sex, causasian/black
  • Standardised body SA
• CKD-EPI
  
  • ​Same variables- slightly different calculation
    
    • As accurate as MDRD when eGFR<60mls/min
    • More accurate than MDRD when eGFR> 60mls/min

Question 19

With which patients might MDRD eGFR be less accurate?

Answer 19

• People w./ kidney disease
• Children
• Pregnancy
• Old age
• Other ethnicities (not caucasian)
• Amputee

Question 20

Why is eGFR less accurate with mild kidney disease?

Answer 20

1. Reduced nephron number- nephron hypertrophy- no change in GFR
2. Reduced GFR so reduced filtration of creatinine causing increased filtration in tubule