8. Measurement of kidney function

Question 1

What does GFR depend on?

Answer 1

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

Question 2

Why is low GFR in babies not a concern?

Answer 2

• Nephron development finished by 35th-36th week of fetal development
• Premature and LBW infants often have lower nephron numbers
• Fetal excretion predominantly via placenta
• At birth GFR – 20ml/min/1.73m2
• Normal GFR by ~ 18 months

Question 3

Why does GFR start declining after 30 years of age?

Answer 3

• Loss of functioning nephrons

* Some compensatory hypertophy

Question 4

How does pregnancy affect GFr?

Answer 4

• GFR increases (~ 50%)
• 130 – 180 ml / minute
• Kidney size increases (~ 1 cm)
• Increased fluid volume (vascular & interstitial)
• Nephron number the same
• Back to pre-pregnancy levels ~ 6 months post-partum

Question 5

What does a decrease in GFR in an individual suggest?

Answer 5

kidney function has worsened
• Decline in number of nephrons
• Decline of GFR within individual nephrons

Question 6

What does a increase in GFR in an individual suggest?

Answer 6

kidney function has recovered

Question 7

In decline of kidney function, why is a decrease in GDR not seen initially?

Answer 7

When kidney function declines slowly, individual nephrons may hypertrophy so actual GFR may not fall until significant kidney damage has occurred

Question 8

Conditions for a substance to be used to measure GFR?

Answer 8

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

Question 9

Why is inulin not used to measure GFR?

Answer 9

• It requires continuous IVI to maintain steady state

* Requires catheter and timed urine collections

Question 10

What is 51 CR-EDTA?

Answer 10

• Radio-active labelled marker
• Cleared exclusively by renal filtration
• Timed injection with blood samples taken 2,3,4 hours afterward

used to measure GFR

Question 11

When is 51 CR-EDTA used to measure GFR?

Answer 11

• In children

* Where indication of renal function required e.g. kidney transplant or work up to donate kidney for transplant

Question 12

What is creatinine?

Answer 12

• Endogenous substance

* End product of muscle breakdown

Question 13

What are the disadvantages of using creatinine to measure GFR?

Answer 13

• Cumbersome – (carrying a bottle of urine) & frequently inaccurate
• Overestimates GFR by 10 – 20% due to creatinine secretion (more with more severe renal impairment)

Question 14

When is creatinine used to measure GFR?

Answer 14

pregnancy

Question 15

What factors can increase serum creatinine levels in an individual?

Answer 15

• Large muscle bulk (Muscle cell breakdown)
• Young
• Male
• Black
• Creatine supplements
• High intake of meat
• Certain drugs e.g. trimethoprim

Question 16

What factors can decrease serum creatinine levels in an individual?

Answer 16

• Reduced muscle mass
• (Muscle cell breakdown)
• Old
• Female
• Hispanic / Indo-Asian
• Vegetarian

Question 17

Is there variation in GFr between individuals?

Answer 17

yes

• Serum creatinine (sCr) stable in an individual (in steady state)
• But serum creatinine can reflect very different glomerular filtration rates in different individuals

Question 18

What is • MDRD eGFR?

Answer 18

• estimated GFR Standardised to body surface area of

1.73 m2 therefore don’t need patient height and weight

Question 19

What factors affect eGFR?

Answer 19

• Serum creatinine
• Age
• Sex
• Caucasian or Black
• Standardised to body sur

Question 20

When is eGFR inaccurate?

Answer 20

• People without kidney disease (e.g. transplant donors)
• Children
• Pregnancy
• Old age
• Other ethnicities
• Amputees / significantly reduced muscle mass
• Patients with higher levels of kidney function (GFR > 60 mls/min)
• When true GFR changes quickly (e.g. AKI)***

Question 21

Why is eGFR inaccurate in mild kidney

disease?

Answer 21

• Reduction in GFR (e.g. if glomerular surface area reduced) causes increases in blood flow)
• Reduced nephron number leads to nephron hypertrophy so no change in GFR
• Reduced filtration of creatinine (due to reduced GFR) results in increased serum creatinine and increased secretion into the tubule (in order to maintain relatively steady state of serum creatinine)