Measuring GFR And Renal Clearance

Question 1

GFR gives us a rough measure of the __________

Answer 1

Number of functioning nephrons

Question 2

How do you calculate the filtered load of a substance?

Answer 2

FL=(GFR)(P)

P=plasma concentration of substance x

Question 3

How do you calculate the excretion rate of substance X?

2 ways

Answer 3

ER=(U)(V)

ER=FL+S- R

U=urine concentration of x
V= urine rate

FL=filtered load
S=secretion rate
R= reabsorption rate

Question 4

The law of conservation of mass applies when a substance is __________

Answer 4

Not secreted and not reabsorbed

What goes in comes out your pee hole

Question 5

How do you calculate GFR using the urine and plasma concentrations of inulin or creatinine?

Answer 5

You just calculate the clearance of inulin or creatinine!
(U)(V)
C=________ =GFR
P

U=urine conc of inulin/creatinine
V=urine rate
P=plasma conc of inulin/creatinine

Question 6

Why can we use the clearance of inulin to calculate GFR

Answer 6

Because inulin is freely filtered and then doesnt get reabsorbed or secreted

Question 7

Since we don’t really use inulin in the clinical setting anymore, what can we use instead to measure GFR?

Answer 7

Creatinine

Question 8

Why do we use creatinine?

Answer 8

• it’s produced by muscle at a constant rate
• it is freely filtered, not reabsorbed, and VERY slightly secreted
• your patient is preloaded with it, you don’t have to give them a dose of inulin or whatever else

Question 9

What are the two ways you can use creatinine to estimate GFR?

Answer 9

1. Calculate the clearance of creatinine to get GFR (requires a 24 hr urine collection)
2. Use the Plasma conc of creatinine and plug it into an algorithm (can be done with a single blood draw)

Question 10

How would you estaimate GFR for your trauma patient that just rolled into the ER without his bag of urine from the last 24 hours?

Answer 10

You would just measure the amount of creatinine in his blood and then plug that number into a standard predictive algorithm that takes into account their weight, age, race, gender etc

Question 11

What are the 3 standard predictive equations used to estimate GFR from plasma creatinine?

Answer 11

1. Cockcroft-Gault
2. MDRD 4
3. CKD epidemiology

Question 12

Your liver produces urea at a rate dependent on several variables, and then is excreted by the kidneys. Why isn’t it a good marker of GFR?

Answer 12

About 1/2 of filtered urea is reabsorbed

Question 13

Is BUN a good measure of GFR?

Answer 13

No, because about half of filtered urea is reabsorbed

Question 14

What does “Clearance” represent?

Answer 14

The volume of plasma that is cleared of solute x per minute

Question 15

How do you calculate the clearance of a substance?

Answer 15

(Ux)(V)
Cx=_________
Px

HiGh YiElD

Question 16

If the clearance of substance x is greater than GFR, is it secreted or absorbed?

Answer 16

Secreted

Question 17

If the clearance of substance X is less than GFR, is it secreted or reabsorbed?

Answer 17

Reabsorbed

Question 18

If the clearance of substance X is equal to GFR, is it secreted or reabsorbed?

Answer 18

Neither, it is freely filtered (and is also a good measure of GFR)

Question 19

What is the clearance of glucose in a healthy person?

Answer 19

0

100% reabsorbed

Question 20

What is a clearance ratio?

Answer 20

It is a ratio of the clearance of X to the clearance of inulin (aka GFR)

Question 21

How do you calculate the clearance ratio?

Answer 21

Cx
_______
C inulin

Question 22

If your clearance ratio is 1, what does that mean?

Answer 22

The substance must also be a GFR marker.

Filtered and neither secreted nor reabsorbed)

Question 23

If your clearance ratio is less than 1, what does that mean?

Answer 23

It means that the substance is either not filtered at all, or it is reabsorbed

(Just means that the clearance of X is less than GFR)

Question 24

If your clearance ratio is greater than 1, what does that mean?

Answer 24

It means that the substance is filtered and secreted

Clearance of X is greater than GFR

Question 25

What measurement will the clearance of PAH give us?

Answer 25

C pah= RPF

Question 26

What happens to PAH in the nephron?

Answer 26

100% of the blood is cleared of PAH in one pass.

All of the PAH that enters the nephron is excreted in the urine

Question 27

What is the difference between RPF and “effective” RPF?

Answer 27

When blood goes into the renal artery, about 10% of it goes to places other than the nephrons, so using PAH clearance to estimate RPF is ignoring that 10% of blood.

So PAH really gives us the “effective” RPF, and not the true RPF, but we don’t really care about that. The effective RPF is good enough for us

Question 28

How do you use RPF to estimate renal blood flow?

Answer 28

RPF
RBF= _______
(1-Hct)

(1-Hct) is just the percentage of blood that is plasma