4. Measurement Of Kindey Function

Question 1

What is glomerular filtration rate?

Answer 1

Amount of filtrate produced from the blood flow to the kidneys per unit time.

Question 2

What is normal GFR in males and females?

Answer 2

Males - 120 mL/min/1.73m^2.

Females - 90 mL/min/1.73m^2.

Question 3

What factors affect GFR?

Answer 3

Gender.
Age.
Size of individual.
Size of kidneys.
Pregnancy.

Question 4

In which weeks of foetal development does nephron development finish?

Answer 4

35-36 weeks.

Question 5

What is GFR at birth?

Answer 5

20 mL/min/1.73m^2.

Question 6

How long after birth does it take for normal GFR to be reached?

Answer 6

18 months.

Question 7

At what age does GFR start to decline and at what rate?

Answer 7

30 years.

6-7 mL/min per decade.

Question 8

Why does GFR decline with age?

Answer 8

Loss of functioning nephrons.

Question 9

How do the kidneys try to adapt to the decline in GFR with an increase in age?

Answer 9

Compensatory hypertrophy, thinned cortex and medulla hypertrophy (still overall decrease in kidney volume).

Question 10

In the case of one hyperplastic kidney, what affect does this have on the other normal kidney?

Answer 10

Compensatory hypertrophy - existing nephrons get bigger as does the healthy kidney. Occurs to a greater extent in childhood. But the nephrons are still working harder and are at greater risk of wearing out - cortical scars.

Question 11

What are small kidneys worse?

Answer 11

In a big person (rather than small person).

Question 12

What happens to GFR in pregnancy?

Answer 12

Increases by approx 50, with a slight increase in kidney size (nephron number stays the same), due to an increased fluid volume.

Question 13

How long does it take GFR to return to pre-pregnancy levels, post-partum?

Answer 13

6 months.

Question 14

What may happen to GFR if kidney function declines slowly?

Answer 14

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

Question 15

What is clearance?

Answer 15

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

Question 16

What is the formula for clearance from the body?

Answer 16

Clearance = amount of substance eliminated from plasma / plasma concentration of substrate

Question 17

What is the formula for excretion rate?

Answer 17

Excretion rate = amount in urine x urine flow rate

Question 18

What is the formula for renal clearance of a substrate?

Answer 18

Renal clearance = excretion rate / plasma concentration

Question 19

What is the criteria for a substance to be used to measure GFR?

Answer 19

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 20

Although insulin clearance is a surrogate for GFR, why don’t we use it in clinical practice?

Answer 20

Requires continuous IVI to maintain steady state.

Requires catheter and timed urine collections.

Question 21

How would you use 51 Cr-EDTA to measure GFR?

Answer 21

Radio-active labelled marker used instead of insulin.
Given as one single timed injection.
Cleared exclusively by renal filtration.
Blood samples taken 2, 3 and 4 hours afterwards.
10% lower clearance than insulin.

Question 22

When is 51 Cr-EDTA used to measure GFR?

Answer 22

In children.

Where indication of renal function is required eg kidney transplant or work up to donate kidney for transplant.

Question 23

What is creatinine?

Answer 23

The end product of muscle breakdown.

Question 24

Why is/isn’t creatinine a good measure of GFR?

Answer 24

SOMETIMES produced at a constant rate.
Is freely filtered across the membrane.
Is not reabsorbed along the nephron.
HOWEVER it is secreted into the nephron from the peritubular capillary, making it an overestimate of GFR.

Question 25

How is creatinine clearance measured clinically?

Answer 25

Collect urine over 24 hours, and measure serum creatinine.

Question 26

What are the disadvantages to using creatinine clearance to measure GFR?

Answer 26

Cumbersome as have to carry a bottle of urine.
Frequently inaccurate.
Overestimates GFR by 10-20% (more with more severe renal impairment).

Question 27

When is creatinine clearance used to estimate GFR?

Answer 27

Pregnancy.

Question 28

What affects creatinine levels in an individual?

Answer 28

Intake, metabolism.

Renal excretion and extra-renal (bowel) excretion.

Question 29

As GFR declines, what makes up proportionately more of the total creatinine excretion?

Answer 29

Renal secretion (from peritubular capillaries to tubule) and extra-renal secretion.

Question 30

Name 3 factors that can cause increased serum creatinine.

Answer 30

Muscle cell breakdown.
Large muscle bulk.
Male.
Black.
Creating supplements.
Meat.
Certain drugs eg trimethoprim.

Question 31

Name 3 factors that can cause reduced serum creatinine.

Answer 31

Reduced muscle mass.
Old.
Hispanic/Indo-Asian.
Female.
Vegetarian.

Question 32

What is eGFR?

Answer 32

GFR calculated using an equation based on creatinine clearance, to take into account serum creatinine level variations.

Question 33

What 4 variables are taken into account in the equation for measuring eGFR?

Answer 33

Serum creatinine.
Age.
Sex.
Caucasian or black.

Question 34

Name 3 groups eGFR is still inaccurate in?

Answer 34

People without kidney disease eg transplant donors.
Children.
Pregnancy.
Old age (overestimate).
Other ethnicities other than Caucasian or black.
Amputees/significantly reduced muscle mass.
Patients with higher levels of kidney function (GFR>60mls/min).
When true GFR changes quickly eg AKI.
Taken from changes in serum creatinine and not all changes are sure to change in GFR!

Question 35

When serum creatinine is close or within normal range, how is the eGFR inaccurate? Why is this a problem?

Answer 35

Underestimates true GFR.

Patient may be inaccurately labelled as having CKD.

Question 36

Why is eGFR less accurate in a patient with mild kidney disease?

Answer 36

Reduction in GFR causes an increase in blood flow.
Reduced nephron number leads to nephron hypertrophy so no change in GFR.
Reduced filtration of creatinine results in increased serum creatinine and so increased secretion into the tubule (so maintains a steady state of serum creatinine).