Assessment of Renal Function Flashcards
How do we assess renal function
Asess through measurement of the GFR:
Measuring the volume of various substances in plasma being cleared by the kidney
What is the equation for plasma clearance of X
CX= [UX] x V/[PX]
units are mls/min
UX = Urine concentration of X, V = urine flow rate, PX = plasma concentration of X
What is the normal GFR
Normal GFR is aprox 100mls/min/1.73m2
Correlates with the surface area and is an average across a range of adults and sexes, for kidney function and size,
What happens to GFR with age
GFR declines by 1ml/min/year after 30 years
What are three substances used for clearance measurement in GFR
Insulin
51Ce-ECTA - radioactive substance
Creatinine
Why is insulin a gold standard measurement in GFR
because Insulin is freely filtered at glomerulus and neither reabsorbed nor secreted, it is not metabolised by the kidney, nor does it interfere with normal renal function so insulin clearance is a direct measure of GFR
How is the insulin measured in GFR
loading IV dose of insulin, allow time to equilibrate, then sample simultaneously plasma and urine (during a timed urine sample)
What is the clearance rate of substances filtered and reabsorbed compared to insulin
So substances filtered and reabsorbed will have a lower clearance than insulin because the Urinary concentration will be less than if only filtered and the plasma concentration higher
What is the clearance rate of substances filtered and secreted compared to insulin
Substances filtered and secreted will have a higher clearance than insulin because will be higher urinary concentration and plasma concentration is lower
What occurs when filtration is greater than excretion
Net reabsorption on X
What occurs when excretion is greater than filtration
next secretion of X
What happens when clearance of X is less than insulin
Net reabsorption of X
What happens when clearance of X is greater than insulin
Net secretion of X
In clinical practise why isn’t insulin no longe used
Is to heavy
How is 51Ce-ECTA used to measure GFR
a suitable radioactive substance, that is handled by the kidney in the same way as insulin
Why isn’t 51Ce-ECTA used much in clinical practise
GFR is usually to complex and expensive to measure as takes several hours and requires injections of isotopes 51Cr-EDTA
What is routinely used in clinical practise to estimate GFR
Creatinine clearance
How is creatinine used to measure GFR
Used as an estimation as in good agreement with insulin clearance
GFR= CIN = CCR CCR = [UCR] V/ [PCR]
Therefore creatinine clearance CCR = the rate of urine loss divided by the plasma concentration.
Therefore GFR is proportional to 1/(PCR) so plasma creatine can be used to estimate GRF
What affect would halving the GFR on the Plasma creatine
you would expect to see a doubling of [PCR] but it is not a linear relationship, need to half GFR before elevation of [PCR]
What is the disadvantage of measuring serum creatinine to measure GFR
Is only an estimation of GFR
Creatinine has cofounding variables
What are factors that affect serum creatinine
Muscle mass: athletes vs malnutrition
Dietary intake: creatine supplements vs vegetarian
Drus: some lead spurious increase
Ketoacidosis: leads to spurious increase
What is the clearance of glucose
zero clearance because all normally reabsorbed
What is clearance of urea
Clearance is less than that of insulin because some urea is reabsorbed
estimated 50ml/min, as 50% of a 100ml of urea is normally reabsorbed
What is clearance of PAH - organic anion para-amino-hippuric acid used to measure
PAH clearance is a measure of all the plasma flowing through the kidneys in a given time
therefore used to measure renal plasma flow
What is the clearance of PAH
PAH is freely filtered at the glomerulus and then the PAH remaining in the plasma is actively secreted into the tubule so that >90% of plasma is cleared of its PAH content in one transit of the kidney
What is the clearance of penicillin
Penicillin has a greater clearance than insulin because filtered and secreted
The ability to measure GFR is particularly useful in what two clinical situations
Patients with renal disease
Many drugs eg digitalis and antibiotics are removed from the body by excretion and filtration
Why is it important to measure GFR in renal disease
Progression of the underlying renal disease process results in nephron destruction and decrease in nephron function
So as total GFR= sum of all filtration by functioning nephrons, therefore the progression of disease would be indicated by the reduction in GFR
Why is it important t measure GFR in certain drugs
When GFR falls, excretion falls so that drug concentration in plasma may rise causing toxicity
therefore may need to adjust dose appropriately to decrease renal function
