Measuring GFR And Renal Clearance Flashcards
GFR gives us a rough measure of the __________
Number of functioning nephrons
How do you calculate the filtered load of a substance?
FL=(GFR)(P)
P=plasma concentration of substance x
How do you calculate the excretion rate of substance X?
2 ways
ER=(U)(V)
ER=FL+S- R
U=urine concentration of x
V= urine rate
FL=filtered load
S=secretion rate
R= reabsorption rate
The law of conservation of mass applies when a substance is __________
Not secreted and not reabsorbed
What goes in comes out your pee hole
How do you calculate GFR using the urine and plasma concentrations of inulin or creatinine?
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
Why can we use the clearance of inulin to calculate GFR
Because inulin is freely filtered and then doesnt get reabsorbed or secreted
Since we don’t really use inulin in the clinical setting anymore, what can we use instead to measure GFR?
Creatinine
Why do we use creatinine?
- 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
What are the two ways you can use creatinine to estimate GFR?
- Calculate the clearance of creatinine to get GFR (requires a 24 hr urine collection)
- Use the Plasma conc of creatinine and plug it into an algorithm (can be done with a single blood draw)
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?
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
What are the 3 standard predictive equations used to estimate GFR from plasma creatinine?
- Cockcroft-Gault
- MDRD 4
- CKD epidemiology
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?
About 1/2 of filtered urea is reabsorbed
Is BUN a good measure of GFR?
No, because about half of filtered urea is reabsorbed
What does “Clearance” represent?
The volume of plasma that is cleared of solute x per minute
How do you calculate the clearance of a substance?
(Ux)(V)
Cx=_________
Px
HiGh YiElD
If the clearance of substance x is greater than GFR, is it secreted or absorbed?
Secreted
If the clearance of substance X is less than GFR, is it secreted or reabsorbed?
Reabsorbed
If the clearance of substance X is equal to GFR, is it secreted or reabsorbed?
Neither, it is freely filtered (and is also a good measure of GFR)
What is the clearance of glucose in a healthy person?
0
100% reabsorbed
What is a clearance ratio?
It is a ratio of the clearance of X to the clearance of inulin (aka GFR)
How do you calculate the clearance ratio?
Cx
_______
C inulin
If your clearance ratio is 1, what does that mean?
The substance must also be a GFR marker.
Filtered and neither secreted nor reabsorbed)
If your clearance ratio is less than 1, what does that mean?
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)
If your clearance ratio is greater than 1, what does that mean?
It means that the substance is filtered and secreted
Clearance of X is greater than GFR
What measurement will the clearance of PAH give us?
C pah= RPF
What happens to PAH in the nephron?
100% of the blood is cleared of PAH in one pass.
All of the PAH that enters the nephron is excreted in the urine
What is the difference between RPF and “effective” RPF?
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
How do you use RPF to estimate renal blood flow?
RPF
RBF= _______
(1-Hct)
(1-Hct) is just the percentage of blood that is plasma
