Measurement of Kidney Function Flashcards
How is kidney functioned measured?
GFR
What is the normal GFR?
90-120 ml/min/1.73m^2
What can effect GFR?
Gender Age Size of individual Size of kidneys Pregnancy
What is GFR at birth?
20 ml/min/1.73m^2
What age to babies have a normal GFR?
18 months
When does GFR start declining?
After 30
What is the rate of age related decline?
6-7 mls/min per decade
What happens to the GFR in pregnancy?
Increases to 130-180 ml/min/1.73m^2
What is clearance?
Volume of plasma cleared of a substance per unit of time
What is the formula for clearance?
Cx = Ax/Px
C = clearance A = amount of substance eliminated from plasma P = plasma concentration of substrate
What is renal clearance?
Renal clearance = excretion rate / plasma concentration
How is excretion rate calculated?
Excretion rate = amount in urine * urine flow rate
What is the formula for renal clearance?
Cx = (Ux*V) / Pax
C = clearance U = amount in urine V = urine flow rate Pa = arterial plasma concentration
What characteristics should a substance have to be used to measure GFR?
Be produced at a constant rate
Be freely filtered across the glomerulus
Not reabsorbed in the nephron
Not secreted into nephron
Is inulin a suitable substance?
Yes
Why is inulin not used to measure GFR?
Requires continuous ivi
Requires catheter and timed urine collections
What is 51 Cr-EDTA?
Radio-active labelled marker
Is 51 Cr-EDTA a suitable substance?
Yes
Cleared exclusively by renal filreation
~10% lower clearance than inulin
Who is 51 Cr-EDTA used for?
Children
Where indication of renal function required
How is 51 Cr-EDTA used?
Timed injection with sample take 2, 3, 4 hours later
Is creatinine a suitable substance
Be produced at a constant rate - yes/no
Be freely filtered across the glomerulus - yes
Not reabsorbed in the nephron - yes
Not secreted into nephron - no
Over estimate of GFR
What is creatinine?
Endogenous product of muscle breakdown
How is creatinine clearance measured?
Urine collection over 24 hours
Measurement of serum creatinine
What factors can affect creatinine levels?
Protein intake
Metabolism
Muscle bulk
Who has an increased serum creatinine?
Large muscle bulk
Male
Black
Who has a reduced serum creatinine?
Reduced muscle mass Old Female Hispanic/indo-asian Vegetarian
How is serum creatinine used?
eGFR
What variables are used to calculate eGFR?
Serum creatinine
Age
Sex
Caucasian or Black
Who is eGFR inaccurate in?
Children
Pregnancy
Elderly
Why is eGFR less accurate with mild kidney disease?
Reduction in GFR causes increase in blood flow
Reduced nephron number leads to nephron hypertrophy
Reduced filtration of creatinine results in increased serum creatinine
