Measuring kidney Function

Question 1

Measurement of renal clearance can provide

Answer 1

information on the renal function

Question 2

What can we measure?

Answer 2

GFR- glomerular filtration rate
RPF_ Renal plasma flow
RBF- Renal blood flow

Question 3

Which substances can renal clearance be measured

Answer 3

Anything and everything
Value depends on how kidney handles it

Question 4

What is used to measure GFR

Answer 4

Inulin

Question 5

Why inulin?

Answer 5

1) freely filtered
2) not reabsorbed
3) not secreted
amount of inulin excreted per minute=amount filtered

Question 6

Formula

Answer 6

GFR= Urineinulin x urine volume/ plasma inulin
per unit time

Question 7

Why clearance

Answer 7

hypothetical volume of plasma that would be completely cleared of the substance per unit time in order to produce amount produced in urine

Question 8

Formula of clearance X

Answer 8

Urine conc of X times by urine volume per minute/ Plasma conc of X
value: mL/min
mmol cancelled eachother out

Question 9

Two types of measuring gfr

Answer 9

inulin clearance
creatinine clearance

Question 10

Creatinine

Answer 10

Creatinine is produced endogenously as a byproduct of muscle metabolism and released at a relatively constant rate.

Unlike inulin, creatinine is not reabsorbed by the kidneys, but there is a small degree of secretion.

Due to this secretion, the clearance of creatinine slightly overestimates GFR by 10-20%.

Despite this overestimation, the ease of using creatinine clearance makes it a practical choice in clinical settings.

Question 11

Inulin

Answer 11

Inulin is the gold standard for measuring GFR because it is freely filtered, not reabsorbed, and not secreted by the kidney.

However, inulin has some drawbacks:

It is not produced endogenously and must be infused intravenously.

It takes at least 45 minutes for plasma inulin concentration to stabilize.

The protocol requires frequent blood and urine samples (every 10-20 minutes), which makes the procedure time-consuming and invasive.

Because of these difficulties, inulin is rarely used in clinical practice.

Question 12

converting rpf to rbf

Answer 12

RPF= UrinaryPAH x Volume of urine/ Plasma conc of PAH

Question 13

What is PAH

Answer 13

Paramino hippuric acid (PAH)

Question 14

RBF

Answer 14

RBF = RPF / (1 - Hct).
HCT is haematocrit (packed cell volume, 40-45% usually)