Clearance, GFR, RPF and Urea

Question 1

Why would we want to know clearance rate?

Answer 1

Used to calculate:

1) . Glomerular FIltration Rate
2) . Plasma Flow Rate

Question 2

How do you calculate clearance rate?

Answer 2

Using the formula:

Cx = [Px] * V / [Ux]

Question 3

What do you use to calculate the clearance rate in regard to GFR?

Answer 3

Inulin or creatinine

-> (creatinine is used most commonly)

Question 4

What is the “gold standard” GFR test and why?

Answer 4

The gold standard test is Inuline clearance because it is not reabsorbed or secreted
- This means all cleared inulin is via the glomerular filtration

Question 5

Why is inulin not routinely used to calculate GFR?

Answer 5

Because it has to be injected intravenously (and it is slightly toxic)

Question 6

What is the pressure definition of GFR?

Answer 6

GFR is due to pressure across the glomerular barrier
- The hydrostatic pressure within the glomerulus is determined by the difference in the fluid entering the afferent arteriole and leaving via the efferent arteriole

Question 7

Why is creatinine clearance preferred to inulin clearance?

Answer 7

Unlike precise GFR measurements involving constant infusions of inulin, creatinine is already at a steady-state concentration in the blood, and so measuring creatinine clearance is much less cumbersome

Question 8

What information do you need to calculate an accurate creatinine clearance?

Answer 8

1) . Plasma creatinine concentration
2) . Urine volume over 24 hours
3) . Urine creatinine concentration

Question 9

What is usually used in the place of creatinine clearance?

Answer 9

Estimated creatine clearance

Question 10

How is estimated creatinine clearance calculated?

Answer 10

Via plasma creatinine concentrations

Question 11

What is the normal reference range for creatinine?

Answer 11

55 - 120 umol / L

Question 12

Why are the units “ ml / min / 1.73m2” used?

Answer 12

Becuase estimated GFR does not account fo body size

- 1.73m2 being the estimated body surface area of an adult with a mass of 63 kg and a height of 1.7m

Question 13

State the normal range of GFR for adult males and then adult females, adjusted for body surface area?

Answer 13

Men: 100 - 130 (average 125) ml/min/1.73m2
Women: 60 - 100 ml/min/1.73m2

Question 14

GFR of male with Stage 1 CKD?

Answer 14

> 90 ml/min/1.73m2

Question 15

GFR of male with Stage 2 CKD?

Answer 15

60 - 89 ml/min/1.73m2

Question 16

GFR of male with Stage 3 CKD?

Answer 16

30 - 59 ml/min/1.73m2

Question 17

GFR of male with Stage 4 CKD?

Answer 17

15 - 29 ml/min/1.73m2

Question 18

GFR of male with Stage 5 CKD?

Answer 18

<15 ml/min/1.73m2

Question 19

Describe 6 causes of a raised endogenous creatinine?

Answer 19

1) . CKD
2) . Certain medications
3) . Kidney Obstruction
4) . Dehydration
5) . Intense exercise or large muscle mass
6) . Dietary (creatinine supplementation or high protein intake)

Question 20

What do we use to calculate renal plasma flow (RPF)?

Answer 20

Para-aminohippuric acid

Question 21

Describe the used of para-aminohippuric acid to calculate RPF?

Answer 21

It is not endogenous so is given as a continuous infusion.

Question 22

How is para-aminohippuric acid cleared?

Answer 22

• 20-30% is filtered

- 70-80% is secreted

Question 23

Why is creatinine clearance inaccurate for GFR?

Answer 23

Because it is also actively secreted by the peritubular capillaries in very small amounts such that creatinine clearance overestimates actual GFR by 10% to 20%

Question 24

What is the common mistake when calculating GFR with creatinine?

Answer 24

Not accounting for muscle mass