Measurement of Kidney Function

Question 1

How is kidney functioned measured?

Answer 1

GFR

Question 2

What is the normal GFR?

Answer 2

90-120 ml/min/1.73m^2

Question 3

What can effect GFR?

Answer 3

Gender
Age
Size of individual 
Size of kidneys 
Pregnancy

Question 4

What is GFR at birth?

Answer 4

20 ml/min/1.73m^2

Question 5

What age to babies have a normal GFR?

Answer 5

18 months

Question 6

When does GFR start declining?

Answer 6

After 30

Question 7

What is the rate of age related decline?

Answer 7

6-7 mls/min per decade

Question 8

What happens to the GFR in pregnancy?

Answer 8

Increases to 130-180 ml/min/1.73m^2

Question 9

What is clearance?

Answer 9

Volume of plasma cleared of a substance per unit of time

Question 10

What is the formula for clearance?

Answer 10

Cx = Ax/Px

C = clearance
A = amount of substance eliminated from plasma 
P = plasma concentration of substrate

Question 11

What is renal clearance?

Answer 11

Renal clearance = excretion rate / plasma concentration

Question 12

How is excretion rate calculated?

Answer 12

Excretion rate = amount in urine * urine flow rate

Question 13

What is the formula for renal clearance?

Answer 13

Cx = (Ux*V) / Pax

C = clearance 
U = amount in urine 
V = urine flow rate
Pa = arterial plasma concentration

Question 14

What characteristics should a substance have to be used to measure GFR?

Answer 14

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

Question 15

Is inulin a suitable substance?

Answer 15

Yes

Question 16

Why is inulin not used to measure GFR?

Answer 16

Requires continuous ivi

Requires catheter and timed urine collections

Question 17

What is 51 Cr-EDTA?

Answer 17

Radio-active labelled marker

Question 18

Is 51 Cr-EDTA a suitable substance?

Answer 18

Yes

Cleared exclusively by renal filreation
~10% lower clearance than inulin

Question 19

Who is 51 Cr-EDTA used for?

Answer 19

Children

Where indication of renal function required

Question 20

How is 51 Cr-EDTA used?

Answer 20

Timed injection with sample take 2, 3, 4 hours later

Question 21

Is creatinine a suitable substance

Answer 21

Be produced at a constant rate - yes/no
Be freely filtered across the glomerulus - yes
Not reabsorbed in the nephron - yes
Not secreted into nephron - no

Over estimate of GFR

Question 22

What is creatinine?

Answer 22

Endogenous product of muscle breakdown

Question 23

How is creatinine clearance measured?

Answer 23

Urine collection over 24 hours

Measurement of serum creatinine

Question 24

What factors can affect creatinine levels?

Answer 24

Protein intake
Metabolism
Muscle bulk

Question 25

Who has an increased serum creatinine?

Answer 25

Large muscle bulk Male Black

Question 26

Who has a reduced serum creatinine?

Answer 26

``` Reduced muscle mass Old Female Hispanic/indo-asian Vegetarian ```

Question 27

How is serum creatinine used?

Answer 27

eGFR

Question 28

What variables are used to calculate eGFR?

Answer 28

Serum creatinine Age Sex Caucasian or Black

Question 29

Who is eGFR inaccurate in?

Answer 29

Children Pregnancy Elderly

Question 30

Why is eGFR less accurate with mild kidney disease?

Answer 30

Reduction in GFR causes increase in blood flow Reduced nephron number leads to nephron hypertrophy Reduced filtration of creatinine results in increased serum creatinine