Measuring Kidney Function

Question 1

what is used to measure kidney function

Answer 1

glomerular filtration rate

Question 2

what is GFR

Answer 2

the amount of filtrate that is produced from the blood flow per unit time

Question 3

what is the normal GFR

Answer 3

90-120 mL/min

Question 4

what is the normal glomerular filtrate per day

Answer 4

140-180 L

Question 5

what does the GFR depend on

Answer 5

gender, age, size of person, size of kidneys, pregnancy

Question 6

how is the GFR different in babies

Answer 6

the GFR is lower as in uterus the mother removed waste products

Question 7

when does a babies GFR become normal

Answer 7

by 18 months

Question 8

how does the GFR change in old age

Answer 8

GFR declines as there is a loss of functioning nephrons

Question 9

what is compensatory hypertrophy

Answer 9

when one kidney enlarges due to a reduction in nephron number in the other kidney

Question 10

with compensatory hypertrophy is there an increase in nephron size or nephron number?

Answer 10

nephron size - although if it occurs early on in development you can get an increase in nephron number

Question 11

does compensatory hypertrophy occur in old age when the kidneys get smaller

Answer 11

no

Question 12

what are the risks of compensatory hypertrophy

Answer 12

the nephrons work hard so may wear out forming cortical scars in the kidney

Question 13

what happens to the GFR in pregnancy

Answer 13

it increases by 50%

Question 14

what does a fall in GFR mean?

Answer 14

kidney function in the individual has worsened

Question 15

what does a rise in GFR mean?

Answer 15

kidney function has recovered

Question 16

what happens to the GFR when kidney function declines slowly

Answer 16

individual nephrons hypertrophy so the GFR may not fall until significant kidney damage occurs

Question 17

what is clearance

Answer 17

the volume of plasma cleared of a substance per unit of time

Question 18

what is the clearance calculation

Answer 18

clearance = amount of substance removed from plasma / plasma concentration of substance

Question 19

what is the calculation for renal clearance

Answer 19

amount of substance in urine x urine volume / plasma concentration

Question 20

what characteristics should a substance being used to measure renal clearance have

Answer 20

• must filter through glomerulus
• not reabsorbed by nephron
• not secreted by nephron
• produced at a constant rate

Question 21

why is inulin not used to measure renal clearance

Answer 21

requires injection by IV, catheterisation and urine monitoring

Question 22

if a substance is completely cleared from the kidney what is it equal to

Answer 22

the GFR

Question 23

what radio active substance is used to measure clearance

Answer 23

51 Cr-EDTA

Question 24

why is clearance 10% with 51 Cr-EDTA

Answer 24

as some is reabsorbed

Question 25

when is 51 Cr-EDTA used

Answer 25

for children

for those undergoing kidney transplant

Question 26

what endogenous substance is used to measure GFR

Answer 26

creatinine

Question 27

why is creatinine not give an accurate GFR

Answer 27

creatinine is also secreted by the nephron so overestimates the GFR by 10-20%

Question 28

how is creatinine clearance measured

Answer 28

by collecting urine over 24 hours and getting a measurement of serum creatinine

Question 29

why is creatinine used in pregnancy

Answer 29

as you don’t want to give a radioactive substance to a pregnant woman

Question 30

what increases creatinine levels

Answer 30

meat, increased muscle mass

Question 31

as the GFR declines what happens to the renal secretion of creatinine

Answer 31

it increases

Question 32

what factors increase serum creatinine

Answer 32

male, black, muscular, meat

Question 33

are serum creatinine levels higher or lower nearly childhood

Answer 33

lower - as the creatinine is the same as the mothers at birth, but as the mother had a higher GFR, more creatinine was filtered

Question 34

how is GFR estimated from serum creatinine

Answer 34

using the eGFR (estimated GFR)

Question 35

why is eGFR less accurate with mild kidney disease

Answer 35

• reduction in GFR
• reduced nephron number leads to nephron hypertrophy so theres no change in GFR
• reduced filtration of creatinine results in increased serum creatinine and increased secretion