2. Measurement Of Kidney Function

Question 1

What does GFR depend on?

Answer 1

Age
Gender
Size of individual
Size of kidneys
Pregnancy

Question 2

What is the GFR at birth?

Answer 2

20ml/min/1.73^2

Question 3

When is a normal GFR reached?

Answer 3

18 months

Question 4

At what age does GFR start declining?

Answer 4

30 years of age

Question 5

What is the rate of decline of GFR?

Answer 5

6-7ml/min per decade

Question 6

How much does GFR increase using pregnancy?

Answer 6

About 50% to 130-180ml/min

Kidney size increases

Question 7

What does a decline inGFR indicate?

Answer 7

Kidney function has worsened

Either decline in number of nephrons or decline of GFR within individual nephrons

Question 8

What happens when kidney function declines slowly?

Answer 8

Individual nephrons may hypertrophy so actual GFR may not fall until significant kidney damage has occurred

Question 9

What is the problem with using inulin to measure GFR?

Answer 9

Requires continuous IV to maintain steady state

Requires catheter and timed urine collections

Question 10

What is used to work out GFR?

Answer 10

51 CR-EDTA

Serum creatinine

Question 11

What are the benefits of 51 CR-EDTA?

Answer 11

Radioactive labelled marker
Cleared exclusively by renal filtration
Timed injection with blood samples taken 2,3,4 hours after

Question 12

When is 51 CR-EDTA used?

Answer 12

In children

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

Question 13

What is creatinine measured by?

Answer 13

Urine creatinine over 24 hours

Serum creatinine

Question 14

What are the disadvantages of creatinine in GFR measurement?

Answer 14

Cumbersome and frequently inaccurate

Overestimates GFR by 10-20% due to creatinine secretion

Question 15

What is normal serum creatinine?

Answer 15

70-150 micromol/L

Question 16

What affects creatinine levels?

Answer 16

Muscle mass
Protein intake
Renal excretion

Question 17

What can increase serum creatinine?

Answer 17

Large muscle bulk
Young
Male 
Black
Creatine supplements
High intake of meat
Certain drugs

Question 18

What can reduce serum creatinine?

Answer 18

Reduced muscle mass
Old
Female
Hispanic/Indo-Asian
Vegetarian

Question 19

What is MDRD eGFR inaccurate in?

Answer 19

People without kidney disease
Children
Pregnancy
Old age
Other ethnicities 
Amputees/significantly reduced muscle mass
Patients with higher levels of kidney function
Mild kidney disease

Question 20

Why is eGFR inaccurate in mild kidney disease?

Answer 20

Reduction in GFR causes increased blood flow
Reduced nephron number leads to nephron hypertrophy so no change in GFR
Reduced filtration of creatinine results in increased serum creatinine and increased secretion into the tubule