4. Measurement Of Kindey Function Flashcards
What is glomerular filtration rate?
Amount of filtrate produced from the blood flow to the kidneys per unit time.
What is normal GFR in males and females?
Males - 120 mL/min/1.73m^2.
Females - 90 mL/min/1.73m^2.
What factors affect GFR?
Gender. Age. Size of individual. Size of kidneys. Pregnancy.
In which weeks of foetal development does nephron development finish?
35-36 weeks.
What is GFR at birth?
20 mL/min/1.73m^2.
How long after birth does it take for normal GFR to be reached?
18 months.
At what age does GFR start to decline and at what rate?
30 years.
6-7 mL/min per decade.
Why does GFR decline with age?
Loss of functioning nephrons.
How do the kidneys try to adapt to the decline in GFR with an increase in age?
Compensatory hypertrophy, thinned cortex and medulla hypertrophy (still overall decrease in kidney volume).
In the case of one hyperplastic kidney, what affect does this have on the other normal kidney?
Compensatory hypertrophy - existing nephrons get bigger as does the healthy kidney. Occurs to a greater extent in childhood. But the nephrons are still working harder and are at greater risk of wearing out - cortical scars.
What are small kidneys worse?
In a big person (rather than small person).
What happens to GFR in pregnancy?
Increases by approx 50, with a slight increase in kidney size (nephron number stays the same), due to an increased fluid volume.
How long does it take GFR to return to pre-pregnancy levels, post-partum?
6 months.
What may happen to GFR if kidney function declines slowly?
Individual nephrons may hypertrophy, so actual GFR may not fall until significant kidney damage has occurred.
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 for clearance from the body?
Clearance = amount of substance eliminated from plasma / plasma concentration of substrate
What is the formula for excretion rate?
Excretion rate = amount in urine x urine flow rate
What is the formula for renal clearance of a substrate?
Renal clearance = excretion rate / plasma concentration
What is the criteria for a substance to be used to measure GFR?
Be produced at a constant rate.
Be freely filtered across the glomerulus.
Not be reabsorbed in the nephron.
Not be secreted into the nephron.
Although insulin clearance is a surrogate for GFR, why don’t we use it in clinical practice?
Requires continuous IVI to maintain steady state.
Requires catheter and timed urine collections.
How would you use 51 Cr-EDTA to measure GFR?
Radio-active labelled marker used instead of insulin.
Given as one single timed injection.
Cleared exclusively by renal filtration.
Blood samples taken 2, 3 and 4 hours afterwards.
10% lower clearance than insulin.
When is 51 Cr-EDTA used to measure GFR?
In children.
Where indication of renal function is required eg kidney transplant or work up to donate kidney for transplant.
What is creatinine?
The end product of muscle breakdown.
Why is/isn’t creatinine a good measure of GFR?
SOMETIMES produced at a constant rate.
Is freely filtered across the membrane.
Is not reabsorbed along the nephron.
HOWEVER it is secreted into the nephron from the peritubular capillary, making it an overestimate of GFR.
How is creatinine clearance measured clinically?
Collect urine over 24 hours, and measure serum creatinine.
What are the disadvantages to using creatinine clearance to measure GFR?
Cumbersome as have to carry a bottle of urine.
Frequently inaccurate.
Overestimates GFR by 10-20% (more with more severe renal impairment).
When is creatinine clearance used to estimate GFR?
Pregnancy.
What affects creatinine levels in an individual?
Intake, metabolism.
Renal excretion and extra-renal (bowel) excretion.
As GFR declines, what makes up proportionately more of the total creatinine excretion?
Renal secretion (from peritubular capillaries to tubule) and extra-renal secretion.
Name 3 factors that can cause increased serum creatinine.
Muscle cell breakdown. Large muscle bulk. Male. Black. Creating supplements. Meat. Certain drugs eg trimethoprim.
Name 3 factors that can cause reduced serum creatinine.
Reduced muscle mass. Old. Hispanic/Indo-Asian. Female. Vegetarian.
What is eGFR?
GFR calculated using an equation based on creatinine clearance, to take into account serum creatinine level variations.
What 4 variables are taken into account in the equation for measuring eGFR?
Serum creatinine.
Age.
Sex.
Caucasian or black.
Name 3 groups eGFR is still inaccurate in?
People without kidney disease eg transplant donors.
Children.
Pregnancy.
Old age (overestimate).
Other ethnicities other than Caucasian or black.
Amputees/significantly reduced muscle mass.
Patients with higher levels of kidney function (GFR>60mls/min).
When true GFR changes quickly eg AKI.
Taken from changes in serum creatinine and not all changes are sure to change in GFR!
When serum creatinine is close or within normal range, how is the eGFR inaccurate? Why is this a problem?
Underestimates true GFR.
Patient may be inaccurately labelled as having CKD.
Why is eGFR less accurate in a patient with mild kidney disease?
Reduction in GFR causes an increase in blood flow.
Reduced nephron number leads to nephron hypertrophy so no change in GFR.
Reduced filtration of creatinine results in increased serum creatinine and so increased secretion into the tubule (so maintains a steady state of serum creatinine).
