Measurement of Kidney Function Flashcards
What does the kidney regulate?
1) Blood pressure
2) Blood volume
3) pH
4) Electrolytes
5) Osmolality
What is the kidney responsible for the excretion of?
Waste products and drugs
What is kidney function measured in terms of?
Glomerular filtration rate
What is GFR?
Amount of filtrate that is produced from the blood flow per unit of time
What is the normal GFR range?
90-120 ml/min/1.73m^2
What is the normal total glomerular filtrate per day?
140-180 L/day
After what age does GFR start to decline?
30 years of age
What is the rate of decline of GFR after the age of 30?
6-7 mls/min per decade
What happens to GFR in pregnancy?
Increases by around 50%
What happens to kidney size and nephron number in pregnancy?
Kidney size increases but nephron number stays the same
What may a decline in GFR be due to?
Decline in the number of nephrons or a decline of GFRR within individual nephrons
Why may actual GFR not fall until significant kidney damage has occurred?
When kidney function declines slowly, there may be initial hypertrophy in individual nephrons
What is clearance?
The volume of plasma cleared of a substance per unit of time where the substance is denoted as ‘x’
What is the formula used to calculate clearance?
Cx = Ax/Px
where A is amount of substance eliminated from plasma and P is plasma concentration of substrate
What is the equation for renal clearance of a substrate?
Renal clearance = excretion rate/plasma concentration
How do you calculate excretion rate?
U(amount of urine) x V(urine flow rate)
If used to measure kidney clearance, what should a substance be?
1) Produced at a constant rate
2) Be freely filtered across the glomerulus
3) Not be reabsorbed into the nephron
4) Not be secreted into the nephron
Why isn’t inulin used to measure GFR?
1) Requires continuous iv to mainstain steady state
2) Requires catheter and timed urine collections
When is 51 Cr-EDTA used in clinical practice?
In children and where indication of renal function is required (eg kidney transplant)
How is creatinine clearance measured?
By collecting urine over 24 hours
Measurement of serum creatinine
What are the limitations of using creatinine to determine renal function?
Frequently inaccurate, overestimates GFR by 10-20% due to creatinine secretion
What things affect creatinine levels in an individual?
Intake (eating meat)
Extra-renal excretion (bowel)
Increases if renal excretion gets worse
What things can cause an increased serum creatinine?
Muscle cell breakdown, large muscle bulk Being male, black Creatine supplements Meat Certain drugs like trimethoprim
What things can cause a reduced serum creatinine?
Reduced muscle mass
Being older, hispanic, indo-asian, female
Vegetarian
What is used in UHL to determine renal function?
MDRD eGFR
4 variable equation, standardised to body surface area of 1.73m^2
When is MDRD eGFR inaccurate?
1) People without kidney disease
2) Children
3) Pregnancy
4) Old age
5) Other ethnicities (non white)
6) Amputees/reduced muscle mass
7) Patients with higher levels of kidney function
Why is eGFR less accurate with mild kidney disease?
1) Reduction in GFR causes increases in blood flow
2) Reduced nephron number leads to nephron hypertrophy so no change in GFR
3) Reduced filtration of creatinine results in increased serum creatinine and increased secretion into the tubule
