Assesment of renal function

Question 1

Why is it important to be able to assess renal function

Answer 1

Because of its vital role in maintaining homeostasis

Question 2

2 clinical situations where measuring GFR would be particularly useful

Answer 2

1. In patients with renal disease, progression of the underlying disease process results in nephron destruction and decreased nephron function.
2. Many drugs are removed by the kidneys. When GFR falls, excretion falls so that [drug] in plasma may rise causing toxicity.

Question 3

Total GFR = ?

Answer 3

Sum of all filtration by functioning nephrons

Question 4

What result of what test could suggest progression of renal disease

Answer 4

Reduced GFR

Question 5

When someone has reduced renal function what may have to be done to their drug doses and why

Answer 5

Doses may have to be reduced as decreased renal function 》decreased excretion 》increased [drug]

Question 6

What body fluid is measured to determine GFR

Answer 6

Plasma

NOT URINE

Question 7

What test accesses GFR

Answer 7

Plasma clearance test

Question 8

Equation for accessing plasma clearance and what each symbol stand for

Answer 8

Cx(subscript) = [Ux(subscript)]  Vx(subscript)/[Px (subscript)]
Cx = Plasma clearance of x
Ux = Urine concentration of X
V = urine flow rate
Px = plasma concentration of X

Question 9

What does plasma clearance mean

Answer 9

The VOLUME of PLASMA cleared

NOT the QUANTITY of SUBSTANCE cleared

Question 10

Gold standard plasma clearance test

Answer 10

Inulin clearance (polyfructose)
Loading iv dose of inulin 》allow time to equilibrate 》sample plasma and urine simultaneously (during a timed urine sample).

Question 11

Why is inulin clearance a good test for GFR

Answer 11

It s freely filtered at the glomerulus
Not reabsorbed
Not secreted
Not metabolized by kidneys
Does not interfere with normal renal function

Question 12

Normal GFR

Answer 12

125 mls/min

Question 13

What is GFR if
Uin = 285 mg/dl
V = 1.1 mls/min
Pin = 2.5 mg/dl

Answer 13

GFR = 285 (1.1/2.5)

=125.4 mls/min

Question 14

What will be the clearance of substances that are filtered and reabsorbed, why

Answer 14

Lower clearance than inulin
[Ux] is lower because x reabsorbed into blood
[Px] will be higher because x is reabsorbed into blood

Question 15

What will be the clearance of substances that are filtered and secreted, why

Answer 15

Higher than inulin
[Ux] will be higher as x is also secreted into urine, not just filtered at glomerulus
[Px] will be lower as x is removed from plasma after glomerulus by secretion

Question 16

Women’s GFR compared to men

Answer 16

~10% lower

Question 17

Why is eGFR measured more routinely than GFR

Answer 17

GFR is usually too complex and expensive to measure

Takes several hours

Question 18

What is measured in eGFR and where does it come from

Answer 18

Creatinine clearance

Muscle breakdown of creatine

Question 19

Factors affecting plasma creatinine

Answer 19

Muscle mass: athletes vs malnutrition
Dietary intake: creatine supplements vs vegetarians
Drugs: Some lead to increases as does KETOACIDOSIS

Question 20

Clearance of glucose, urea and penicillin compared to inulin

Answer 20

Glucose = 0 (all reabsorbed)
Urea = < inulin (some reabsorbed)
Penicillin = > (filtered and secreted)

Question 21

What’s RPF

Answer 21

Real plasma flow

Question 22

What’s used to measure RPF

Answer 22

Para-amino-hippuric acid (PAH) clearance

Question 23

Value of RPF

Answer 23

660 mls/min

Question 24

What happens to PAH at the glomerulus

Answer 24

It is freely filtered
PAH remaining in the plasma is secreted into the tubule ( >90% of plasma is cleared of its PAH in one transit to the kidney)

Question 25

What is PAH clearance a measure of

Answer 25

All the plasma flowing through the kidneys in a given time

Question 26

Result when;
Filtration > excretion
Excretion > filtration
Filtration = excretion

Answer 26

Net reabsorption of x
Net secretion of x
No Net secretion or reabsorption of x