6 - GFR and Clearance

Question 1

What is GFR and what is the normal values for this?

Answer 1

• Amount of filtrate produced from the blood flow per unit time
• 90 to 120 mL/min

125 mls/min

Question 2

What does GFR depend on?

Answer 2

• Babies have lower GFR due to maturing of kidney. Normalises from 20ml/min to normal by 18 months
• Over 30 GFR starts to decline as loss of functioning nephrons, there is compensatory hypertrophy so increase in medulla volume
• Pregnancy GFR increases as hypertrophy of kidney 130-180ml/min

Question 3

How is GFR a measure of kidney function?

Answer 3

- Decrease: function declined as either drop of nephrons (e.g nephrotoxic drugs) or drop of GFR in individual nephrons (e.g glomerulonephritis)

- Increase: kidney function recovered

• If kidney function declines slowly may be no drop in GFR due to nephron hypertrophy so won’t show on GFR until significant damage

Question 4

What is clearance?

Answer 4

• Volume of plasma cleared of a substance per unit of time
• Encompasses renal and extra-renal clearance

Question 5

What is renal clearance?

Answer 5

Question 6

If urine flow or plasma concentration increases, what happens to renal clearance?

Answer 6

- Urine flow: clearance would increase

- Plasma concentration: clearance will decrease

Question 7

What properties does a substance need to have to measure GFR?

Answer 7

Question 8

What substance is gold standard for measuring GFR?

Answer 8

• Inulin injection that fits all the criteria apart from being produced at a constant rate

Question 9

What other exogenous marker can be used to measure eGFR but not everyday?

Answer 9

• 51 Cr-EDTA
• Only need one injection and blood not urine samples
• Can’t be used every day due to radiation

Question 10

What endogenous substance is used to measure GFR?

Answer 10

Question 11

Why does using creatinine only provide us with an estimate of GFR?

Answer 11

• Overestimate by 10-15% as there is secretion into tubule
• Different factors affect your creatinine levels e.g age, diet, ethnicity and activity levels

Question 12

How is creatinine clearance measured?

Answer 12

• Used in pregnancy as no radiation
• May forget to fill urine bottle every time

Question 13

What is the normal levels of creatinine in the body and how do you convert between American and UK units?

Answer 13

Question 14

How direct is the relationship between true GFR and serum creatinine?

Answer 14

Serum 130 creatinine would be anywhere from 20 to 90 mL/min so we call it eGFR

Question 15

What is the method to calculate eGFR in UHL and Lincolnshire?

Answer 15

• UHL MDRD (inaccurate in children, pregnancy, old age, other ethnicities to black and caucasian, amputees, patients with higher levels of kidney function)
• CKD-EPI (more accurate than MDRD when eGFR over 60mls/min)

Quick change in GFR may be AKI

Question 16

Why is eGFR less accurate in mild kidney disease?

Answer 16

Poor clearance means poor renal function

Question 17

What would happen to P_GC if the afferent arteriole relaxed but the efferent arteriole tone stayed the same?

Answer 17

• Would increase
• Think about queue of people and funnel (Cathie)

Question 18

Label the following diagram

Answer 18

Question 19

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Question 20

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Question 21

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Question 22

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Question 23

A male patietnt with renal disease has an extended period of bed rest. Tests reveal and abnormally high concentration of creatinine in his blood but lower than normal in his urine, what is going on?

Answer 23

Decreasing kidney function

Question 24

What can be some causes of acute kidney injury?

Answer 24

• Common in hospitals, look and U+Es and GFR
• Hypotension secondary to blood loss or sepsis
• Dehydration without hypotension

Question 25

Answer 25