urinary concentration of creatinine mcq

[absolutjag9]

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Consider a patient with a glomerular filtration rate of 60 ml/min, and a urine flow rate of 1 ml/min. Given the way that creatinine is handled by the kidney, you would expect the urinary concentration of creatinine to be:
A. approximately equal to the plasma concentration of creatinine.
B. about 10% greater than the plasma concentration of creatinine. x
C. twice the plasma concentration of creatinine.
D. 60x the plasma concentration of creatinine.
E. impossible to estimate from the data available.

apparently answer is d...i am confused. so any help is appreciated.

here's the way i thought of it: creatinine clearance is normally overestimated by 10% due to the fact that some of it is secreted actively by peritubular capillaries. so we know this dude's GFR is lower than normal (normal=120ml/min), so this is the reason why the answer is not B. Clearance of creatinine=urinary concentration of creatinine times urine flow divided by plasma concentration of creatinine. his gfr is allowing for reabsportion of more solutes, which i think leads to less of them in the urine. in terms of creatinine, would more show up in the urine as a result of the increased plasma concentrations over time?

 

[Virtus]

The kidney is concentrating what is being filtered 60 times, so the urinary concentration is going to be 60 times that of the plasma concentration given that creatinine isnt really taken back out.

Maybe my reason is too simplistic, some of the other answers are a bit over my head!

First post, whoop!

 

[hanky1982]

I think this might help...

GFR ~ = Cr Clearance.... (forget the 10% for right now and follow)

Clearance = UV/P
U= Urine [ ], V= Flow rate, P = Plasma [ ]

THUS:
60 =(Ucr * Vcr)/Pcr
We know V=1, therefore the ratio of the urine concentration to plasma concentration must be 60.

 

[absolutjag9]

thanks very much! that's what i did after posting that q 😳

btw nice avatar! LOL

 

[NRAI2001]

Consider a patient with a glomerular filtration rate of 60 ml/min, and a urine flow rate of 1 ml/min. Given the way that creatinine is handled by the kidney, you would expect the urinary concentration of creatinine to be:
A. approximately equal to the plasma concentration of creatinine.
B. about 10% greater than the plasma concentration of creatinine. x
C. twice the plasma concentration of creatinine.
D. 60x the plasma concentration of creatinine.
E. impossible to estimate from the data available.

apparently answer is d...i am confused. so any help is appreciated.

WIth the given GFR in the question x the given urine flow rate = 60 x1 =60; creatinine should normally be about 1 at plasma..... urine creatinine s 60.

BUT ...if we know that the normal plasma concentration should be 120 (as the OP said it should be) a GFR of 60 = 1/2 of normal so plasma concentration should be 2ml ??? So the ratio would be 60:2 wouldnt it?? Obviously not in the answer choices; if the answer said 60x that of a normal person the answer would make more sense? but it is 30x this specific person??

 

[HospitalDO]

I guess "C" to be the answer to the multiple choice!

 

[marktwain]

don't think beyond what is provided. only use what you have been given and make no assumptions.

GFR X Plasma conc = Urine conc X Urine flow rate

GFR= 60ml/min
Urine flow rate= 1ml/min

Plug in the values
60 X Plasma conc= 1 X Urine conc therefore Urine conc is equal to 60times plasma conc