Measurement of Kidney function

Question 1

What does the kidney regulate and what else does it do?

Answer 1

BP
Blood volume
pH
Electrolytes
Osmolality

Excretion of waste products and drugs

Metabolism of drugs, hormones and proteins

Endocrine - makes 1-alpha calcidol, renin, erythropoietin.

Question 2

What is GFR?

Answer 2

Glomerular Filtration Rate.
The amount of filtrate produced from the blood flow per unit time.
The amount of filtrate is determined by the product of the average filtration of each nephron in each kidney.

Question 3

What is a normal GFR?

Answer 3

90-120ml/min (lower end for women)
OR
140-180L/day

Question 4

What does GFR depend on?

Answer 4

Gender 
Age
Size of individual
Size of kidneys
Pregnancy

Question 5

When does nephron development finish?

Answer 5

35-36 weeks = premature = lower nephron number as foetal excretion is predominantly via placenta.

Question 6

How long after birth does GRF become normal?

Answer 6

18 months

Question 7

When does kidney function start declining?

Answer 7

After 30yrs old - decline 6-7ml per decade

Medulla stays same but cortex decreases in size. This is the important bit.

Question 8

What impact does your size have on your kidneys?

Answer 8

Bigger person = bigger kidneys = more nephrons.

Small kidneys in a big person is worse than small kidney in a small person.

Question 9

What happens if you have a reduced nephron number?

Answer 9

Compensatory hypertrophy. This is when your existing nephrons get bigger to compensate for the reduced amount of nephrons.

Healthy kidneys can also get bigger and this occurs to much greater extent in childhood.

Question 10

What are the consequences of compensatory hypertrophy?

Answer 10

Nephrons work harder and are at greater risk of wearing out.

Question 11

How does GFR change in pregnancy?

Answer 11

GRF increases (50%) - 130-180ml/min

Kidney size increases by about 1cm - increased fluid volume (vascular and interstitial).

BUt, nephron number is the same and it goes back to pre-pregnancy levels 6 months postpartum.

Question 12

What do you look at when looking at GFR?

Answer 12

Look at if it changes day to day for that person or if normal for that person because GFR is greatly variable.

Question 13

Why might GFR decline?

Answer 13

No of nephrons

Decline of GFR within nephrons.

Question 14

What does a fall / rise in GFR mean?

Answer 14

Fall = worse 
Rise = recovered

Question 15

What is clearance?

Answer 15

Clearance is the volume of plasma cleaned of a substance per unit of time where the substance is denoted as ‘X’

Question 16

What is the formula for clearance?

Answer 16

C = A/P

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

Question 17

What is the formula for renal clearance?

Answer 17

C = UxV(excretion rate) / P

C = Clearance 
U = amount of urine 
V = urine flow rate
P= arterial plasma concentration

Question 18

What characteristics must the substance measured in the urine have to be an acurate representation of GFR?

Answer 18

Be produced at a constant rate
Be freely filtered across the glomerulus
Not be reabsorbed in the nephron
Not be secreted into the nephron
If all true then excretion rate = GFR

Question 19

Why dont we use inulin to measure kidney function?

Answer 19

• Requires a constant IV to maintain a steady state

- Requires a catheter and timed urine collections.

Question 20

What do we used to measure GFR as a gold standard (not used in all cases)?

Answer 20

51 Cr-EDTA
-This is a radio-active labelled marked that is cleared exclusively by renal filtration.
-You have timed injections with blood samples taken 2,3,4 hours afterwards.
It is used clinically in children and where indication of accurate renal function is required e.g. transplant.

Question 21

What do we normally used to measure GFR in pregnancy?

Answer 21

Creatinine clearance (as can’t do either of the other methods)

Question 22

Why do we use creatinine to measure?

Answer 22

Produced at a constant rate
Freely filtered
Not reabsorbed
Not secreted into nephron (not perfect as should be)

Question 23

Why dont we use creatinine clearance in everyone?

Answer 23

Collect 24 hours of urine so inconvenient which makes the results inaccurate as it decreases compliance.

Question 24

What things can affect individual levels of creatinine?

Answer 24

Intake (meat)
Metabolism
Extra-renal excretion 
Renal excretion (also secretion)
Skin colour - black = more and hispanic / into-asian = less
Age (old = less)

Question 25

In who is MDRD eGFR inaccurate in?

Answer 25

People without kidney disease
Children
Pregnancy
Old age
Other ethnicities
Amputees / reduced muscles mass
Patients with higher level of kidney function 
When TRUE GFR changed quickly (AKI)

Inaccurate in lots of people s only based on study with 1600 people with little variation and no extremes.

Question 26

What is used when GFR is better?

Answer 26

CKD-EPI

This uses more patients and patients from a more diverse background

Question 27

Why is eGFR less accurate with mild kidney disease?

Answer 27

3 contributing factors

• Reduction in GFR 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 increases serum creatinine and increased secretions into the tubule.

Question 28

What is inulin?

Answer 28

A polisaccaride in fruit and veg where none is absorbed and none is excreted.

Question 29

What is normally used to measure GFR?

Answer 29

Serum creatinine