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#1
02-23-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 487 Threads: 95 Thanked 1,466 Times in 358 Posts Reputation: 1476
Calculate the filtration fraction

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....
 The above post was thanked by: apx85 (02-24-2011), Mondoshawan (02-23-2011), pnreddy (02-23-2011), riya rai (12-21-2011)

#2
02-23-2011
 USMLE Forums Master Steps History: 1+CK+CS Posts: 590 Threads: 31 Thanked 1,233 Times in 411 Posts Reputation: 1257
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!
 The above post was thanked by: aktorque (02-23-2011), apx85 (02-24-2011), Doc4Step1 (06-12-2012), DoctorB (02-24-2011), doctorsmonsters (02-23-2011), donofitaly (02-24-2011), patelMD (02-24-2011), riya rai (12-21-2011), rsin21 (02-24-2011), wimzie (02-25-2011)
#3
02-23-2011
 USMLE Forums Guru Steps History: Step 1 Only Posts: 487 Threads: 95 Thanked 1,466 Times in 358 Posts Reputation: 1476

Quote:
 Originally Posted by Mondoshawan B. 20% FF would be GFR/renal plasma flowwe 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
 The above post was thanked by: apx85 (02-24-2011), DoctorB (02-24-2011), Mondoshawan (02-23-2011)

 Tags Physiology-, Renal-, Step-1-Questions

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