Measurement of Kidney Function

Question 1

What does the kidney regulate?

Answer 1

1) Blood pressure
2) Blood volume
3) pH
4) Electrolytes
5) Osmolality

Question 2

What is the kidney responsible for the excretion of?

Answer 2

Waste products and drugs

Question 3

What is kidney function measured in terms of?

Answer 3

Glomerular filtration rate

Question 4

What is GFR?

Answer 4

Amount of filtrate that is produced from the blood flow per unit of time

Question 5

What is the normal GFR range?

Answer 5

90-120 ml/min/1.73m^2

Question 6

What is the normal total glomerular filtrate per day?

Answer 6

140-180 L/day

Question 7

After what age does GFR start to decline?

Answer 7

30 years of age

Question 8

What is the rate of decline of GFR after the age of 30?

Answer 8

6-7 mls/min per decade

Question 9

What happens to GFR in pregnancy?

Answer 9

Increases by around 50%

Question 10

What happens to kidney size and nephron number in pregnancy?

Answer 10

Kidney size increases but nephron number stays the same

Question 11

What may a decline in GFR be due to?

Answer 11

Decline in the number of nephrons or a decline of GFRR within individual nephrons

Question 12

Why may actual GFR not fall until significant kidney damage has occurred?

Answer 12

When kidney function declines slowly, there may be initial hypertrophy in individual nephrons

Question 13

What is clearance?

Answer 13

The volume of plasma cleared of a substance per unit of time where the substance is denoted as ‘x’

Question 14

What is the formula used to calculate clearance?

Answer 14

Cx = Ax/Px

where A is amount of substance eliminated from plasma and P is plasma concentration of substrate

Question 15

What is the equation for renal clearance of a substrate?

Answer 15

Renal clearance = excretion rate/plasma concentration

Question 16

How do you calculate excretion rate?

Answer 16

U(amount of urine) x V(urine flow rate)

Question 17

If used to measure kidney clearance, what should a substance be?

Answer 17

1) Produced at a constant rate
2) Be freely filtered across the glomerulus
3) Not be reabsorbed into the nephron
4) Not be secreted into the nephron

Question 18

Why isn’t inulin used to measure GFR?

Answer 18

1) Requires continuous iv to mainstain steady state

2) Requires catheter and timed urine collections

Question 19

When is 51 Cr-EDTA used in clinical practice?

Answer 19

In children and where indication of renal function is required (eg kidney transplant)

Question 20

How is creatinine clearance measured?

Answer 20

By collecting urine over 24 hours

Measurement of serum creatinine

Question 21

What are the limitations of using creatinine to determine renal function?

Answer 21

Frequently inaccurate, overestimates GFR by 10-20% due to creatinine secretion

Question 22

What things affect creatinine levels in an individual?

Answer 22

Intake (eating meat)
Extra-renal excretion (bowel)
Increases if renal excretion gets worse

Question 23

What things can cause an increased serum creatinine?

Answer 23

Muscle cell breakdown, large muscle bulk
Being male, black
Creatine supplements
Meat
Certain drugs like trimethoprim

Question 24

What things can cause a reduced serum creatinine?

Answer 24

Reduced muscle mass
Being older, hispanic, indo-asian, female
Vegetarian

Question 25

What is used in UHL to determine renal function?

Answer 25

MDRD eGFR

4 variable equation, standardised to body surface area of 1.73m^2

Question 26

When is MDRD eGFR inaccurate?

Answer 26

1) People without kidney disease
2) Children
3) Pregnancy
4) Old age
5) Other ethnicities (non white)
6) Amputees/reduced muscle mass
7) Patients with higher levels of kidney function

Question 27

Why is eGFR less accurate with mild kidney disease?

Answer 27

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