Session 2 - Measuring Kidney Function Flashcards
what is used to measure kidney function
glomerular filtration rate
what is GFR
the amount of filtrate that is produced from the blood flow per unit time
what is the normal GFR
90-120 mL/min
what is the normal glomerular filtrate per day
140-180 L
what does the GFR depend on
gender, age, size of person, size of kidneys, pregnancy
how is the GFR different in babies
the GFR is lower as in uterus the mother removed waste products
when does a babies GFR become normal
by 18 months
how does the GFR change in old age
GFR declines as there is a loss of functioning nephrons
what is compensatory hypertrophy
when one kidney enlarges due to a reduction in nephron number in the other kidney
with compensatory hypertrophy is there an increase in nephron size or nephron number?
nephron size - although if it occurs early on in development you can get an increase in nephron number
does compensatory hypertrophy occur in old age when the kidneys get smaller
no
what are the risks of compensatory hypertrophy
the nephrons work hard so may wear out forming cortical scars in the kidney
what happens to the GFR in pregnancy
it increases by 50%
what does a fall in GFR mean?
kidney function in the individual has worsened
what does a rise in GFR mean?
kidney function has recovered
what happens to the GFR when kidney function declines slowly
individual nephrons hypertrophy so the GFR may not fall until significant kidney damage occurs
what is clearance
the volume of plasma cleared of a substance per unit of time
what is the clearance calculation
clearance = amount of substance removed from plasma / plasma concentration of substance
what is the calculation for renal clearance
amount of substance in urine x urine volume / plasma concentration
what characteristics should a substance being used to measure renal clearance have
- must filter through glomerulus
- not reabsorbed by nephron
- not secreted by nephron
- produced at a constant rate
why is inulin not used to measure renal clearance
requires injection by IV, catheterisation and urine monitoring
if a substance is completely cleared from the kidney what is it equal to
the GFR
what radio active substance is used to measure clearance
51 Cr-EDTA
why is clearance 10% with 51 Cr-EDTA
as some is reabsorbed
when is 51 Cr-EDTA used
for children
for those undergoing kidney transplant
what endogenous substance is used to measure GFR
creatinine
why is creatinine not give an accurate GFR
creatinine is also secreted by the nephron so overestimates the GFR by 10-20%
how is creatinine clearance measured
by collecting urine over 24 hours and getting a measurement of serum creatinine
why is creatinine used in pregnancy
as you don’t want to give a radioactive substance to a pregnant woman
what increases creatinine levels
meat, increased muscle mass
as the GFR declines what happens to the renal secretion of creatinine
it increases
what factors increase serum creatinine
male, black, muscular, meat
are serum creatinine levels higher or lower nearly childhood
lower - as the creatinine is the same as the mothers at birth, but as the mother had a higher GFR, more creatinine was filtered
how is GFR estimated from serum creatinine
using the eGFR (estimated GFR)
why is eGFR less accurate with mild kidney disease
- 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
