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#### aktorque

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The following measurements were taken from a healthy 33-year-old volunteer.

Samples ------- Urine ----------Serum
Creatinine -----200.0 mg/dl ----2.0 mg/dl
Glucose --------0 -------------80.0 mg/dl
Potassium -----50.0 mEq/L------5.0 mEq/L
Uric acid ------15.0 mg/dl -----3.0 mg/dl
PAH ---------100mg/ml -------0.2mg/ml

Which of the following is the best estimate of the filtration fraction in this patient assuming a urine flow of 1.0 ml/mm?

A. 10%
B. 20%
C. 30%
D. 40%
E. 50%

Let me see your math skill....

#### Mondoshawan

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590 Posts
B

B. 20%

FF would be GFR/renal plasma flow
• we can get a rough GFR with (urine creatinine/plasma creatinine)*flow = 100
• we can get renal plasma flow with (urine PAH/plasmaPAH)*flow = 500
• 100/500 = 20%

I guess we could say FF =
[urine creatinine][plasma PAH]/
[plasma creatinine][urine PAH]
=
(200)(.2)/(100)(2) = 40/200 = 1/5 = 20%
... but while I can remember the first two equations, the combined one somehow just doesn't stick. I love math, and I love medicine...but having to do even simple math on a medicine exam somehow makes me nervous!

#### aktorque

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487 Posts
Discussion Starter · ·
B. 20%

FF would be GFR/renal plasma flow
• we can get a rough GFR with (urine creatinine/plasma creatinine)*flow = 100
• we can get renal plasma flow with (urine PAH/plasmaPAH)*flow = 500
• 100/500 = 20%

I guess we could say FF =
[urine creatinine][plasma PAH]/
[plasma creatinine][urine PAH]
=
(200)(.2)/(100)(2) = 40/200 = 1/5 = 20%
... but while I can remember the first two equations, the combined one somehow just doesn't stick. I love math, and I love medicine...but having to do even simple math on a medicine exam somehow makes me nervous!
Honestly, I learned something from you today. Its very interesting to know that we can combine two separate equations into one simple equation. Next time I'll use that simple one instead of using the two... Thx

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