Lecture 7: Measurement of kidney function

Question 1

What does the GFR depend on?

Answer 1

• age (nephron development finished by 35-36th week of fetal development)
• gender
• size of individual
• size of kidneys
• pregnancy

Question 2

What happens to the nephrons in premature and low birth weight infants?

Answer 2

They often have lower nephron numbers

Question 3

How do fetuses excrete?

Answer 3

Via the placenta, so until they are born the nephron number isn’t particularly important

Question 4

What is the GFR at birth?

Answer 4

20 mL/min

Develop a normal GFR around 18 months

Question 5

What happens to your GFR after you turn 30 years old?

Answer 5

GFR starts declining
6-7ml/min per decade
Due to loss of functioning nephrons , however we don’t see this right away due to compensatory hypertrophy

Question 6

What happens to the volume of the medulla as you age?

Answer 6

Because the cortex volume decreases, the volume of the medulla increases to compensate for that

Question 7

What happens to the GFR in pregnancy?

Answer 7

GFR increases by 50% (130-150 ml/min)
-number of nephrons stay the same
-kidney size increases by 1 cm: increased ECF volume
GFR drops back to normal at around 6 months post-partum

Question 8

What should a substance be to ensure that their GFR=excretion rate?

Answer 8

• produced at a constant rate
• be freely filtered across the glomerulus
• not be reabsorbed in the nephron
• not be secreted into the nephron
  e. g. inulin

Question 9

What substance do we use to give an estimate of GFR and what are the drawbacks?

Answer 9

Inulin: plant polysaccharide that is ingested into the body

• requires continuous IV to maintain a steady state
• requires catheter and timed urine collections

Question 10

What marker is used instead of inulin to measure the GFR?

Answer 10

51 Cr-EDTA (exogenous)

• radioactive labelled marker
• cleared exclusively by renal function
• timed injections with blood samples taken 2,3,4 hours afterwards

Question 11

What is an endogenous substance used to measure GFR?

Answer 11

Creatinine: end product of muscle breakdown
-freely filtered
-not reabsorbed in nephron
-producing at a constant rate is variable
But it is secreted into the nephron, therefore it tends to overestimate the GFR by 10-20%

-used in pregnancy

Question 12

How is creatinine levels measured?

Answer 12

Serum (blood) creatinine sample

Urine creatinine over 24 hours

Question 13

What is a normal serum creatinine?

Answer 13

70-150 micromoles/L

Question 14

What affects creatinine levels in an individual?

Answer 14

• muscle mass
• intake of meat
• renal excretion

Question 15

What increases serum creatinine levels?

Answer 15

• large muscle bulk
• young
• black
• male
• creatine supplements
• high intake of meat
• certain drugs e.g. trimethoprim

Question 16

What reduces serum creatinine levels?

Answer 16

• reduced muscle mass
• old
• female
• hispanic/indo-asian
• vegetarian

Question 17

What is a drawback with using serum creatinine?

Answer 17

Have to consider the patient as a whole
Serum creatinine could be the same for some people, however they could be very different people, making it abnormal in some of those people
-serum creatinine can reflect very different glomerular filtration rate in different individuals
-creatinine varies greatly between different individuals

Question 18

What happens to serum creatinine when GFR declines?

Answer 18

When GFR is normal, you have normal levels of serum creatinine
When GFR is lower, you have large variations in serum creatinine

Question 19

How do you estimate the eGFR from serum creatinine?

Answer 19

MDRD
-based on serum creatinine, age, sex, caucasian/black
-standardised to body surface area of patient
CKD-EPI
-uses same variables as above but equation is quite different
-as accurate at MDRD when eGFR<60 ml/min, but more accurate when eGFR>60 ml/min

Question 20

In who is MDRD eGFR inaccurate?

Answer 20

• people without kidney disease
• children
• pregnancy
• old age
• other ethnicities
• amputees

Question 21

Why is eGFR inaccurate in mild kidney disease?

Answer 21

• reduced GFR, causing an increase in blood flow to increase GFR
• reduced nephron number leading to hypertrophy so no change in GFR
• reduced filtration of creatinine results in increased serum creatinine, so therefore there is an increased secretion into the tubule