Assessment of Renal Function

Question 1

How do we assess renal function

Answer 1

Asess through measurement of the GFR:

Measuring the volume of various substances in plasma being cleared by the kidney

Question 2

What is the equation for plasma clearance of X

Answer 2

CX= [UX] x V/[PX]
units are mls/min

UX = Urine concentration of X, 
V = urine flow rate, 
PX = plasma concentration of X

Question 3

What is the normal GFR

Answer 3

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,

Question 4

What happens to GFR with age

Answer 4

GFR declines by 1ml/min/year after 30 years

Question 5

What are three substances used for clearance measurement in GFR

Answer 5

Insulin

51Ce-ECTA - radioactive substance

Creatinine

Question 6

Why is insulin a gold standard measurement in GFR

Answer 6

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

Question 7

How is the insulin measured in GFR

Answer 7

loading IV dose of insulin, allow time to equilibrate, then sample simultaneously plasma and urine (during a timed urine sample)

Question 8

What is the clearance rate of substances filtered and reabsorbed compared to insulin

Answer 8

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

Question 9

What is the clearance rate of substances filtered and secreted compared to insulin

Answer 9

Substances filtered and secreted will have a higher clearance than insulin because will be higher urinary concentration and plasma concentration is lower

Question 10

What occurs when filtration is greater than excretion

Answer 10

Net reabsorption on X

Question 11

What occurs when excretion is greater than filtration

Answer 11

next secretion of X

Question 12

What happens when clearance of X is less than insulin

Answer 12

Net reabsorption of X

Question 13

What happens when clearance of X is greater than insulin

Answer 13

Net secretion of X

Question 14

In clinical practise why isn’t insulin no longe used

Answer 14

Is to heavy

Question 15

How is 51Ce-ECTA used to measure GFR

Answer 15

a suitable radioactive substance, that is handled by the kidney in the same way as insulin

Question 16

Why isn’t 51Ce-ECTA used much in clinical practise

Answer 16

GFR is usually to complex and expensive to measure as takes several hours and requires injections of isotopes 51Cr-EDTA

Question 17

What is routinely used in clinical practise to estimate GFR

Answer 17

Creatinine clearance

Question 18

How is creatinine used to measure GFR

Answer 18

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

Question 19

What affect would halving the GFR on the Plasma creatine

Answer 19

you would expect to see a doubling of [PCR] but it is not a linear relationship, need to half GFR before elevation of [PCR]

Question 20

What is the disadvantage of measuring serum creatinine to measure GFR

Answer 20

Is only an estimation of GFR

Creatinine has cofounding variables

Question 21

What are factors that affect serum creatinine

Answer 21

Muscle mass: athletes vs malnutrition

Dietary intake: creatine supplements vs vegetarian

Drus: some lead spurious increase

Ketoacidosis: leads to spurious increase

Question 22

What is the clearance of glucose

Answer 22

zero clearance because all normally reabsorbed

Question 23

What is clearance of urea

Answer 23

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

Question 24

What is clearance of PAH - organic anion para-amino-hippuric acid used to measure

Answer 24

PAH clearance is a measure of all the plasma flowing through the kidneys in a given time

therefore used to measure renal plasma flow

Question 25

What is the clearance of PAH

Answer 25

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

Question 26

What is the clearance of penicillin

Answer 26

Penicillin has a greater clearance than insulin because filtered and secreted

Question 27

The ability to measure GFR is particularly useful in what two clinical situations

Answer 27

Patients with renal disease

Many drugs eg digitalis and antibiotics are removed from the body by excretion and filtration

Question 28

Why is it important to measure GFR in renal disease

Answer 28

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

Question 29

Why is it important t measure GFR in certain drugs

Answer 29

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