BUN:Cr ratio

[Anonymous]

This is a very high yield concept that I learned from Dr. Goljan who guarenteed was going to show up on the exam....lets just say that Pappy is always right :wink:

The BUN and Creatine numbers are not as important as the ratio. The ratio should be around 10:1 normally. If the kidneys fail, the BUN will increase much more than the creatine. But if something else is causing the azotemia (increased BUN), then the ratio will remain normal because the kidney is fine and still responsible for this ratio. What can cause this? Anything that decreases flow to the kidneys (hypovolumia, MI, etc).

According to Goljan, if BUN:Cr ratio is <15, it is due to some prerenal cause (not the kidney b/c able to maintain ratio). If it is >15, the kidneys are the problem b/c cannot maintain ratio. This very simple concept, which I never really understood until Goljan explained it, can score you some points on the exam. Here is an example.

A 75 yo male nursing home pt has been hospitalized for 2 weeks for pneumonia and is now recovering and ready for discharge.
Labs show:
WBC 8,000
Temp: 98.6
BUN: 12.0
Cr: 1.2

There are no available beds in the nursing home facility and the bedridden pt who cannot care for himself remains in the hospital until arrangements can be made. 2 days later, the labs show a BUN of 40 and Cr of 4.2. What most likely caused the BUN and Cr changes?

a. Renal Failure
b. renal artery stenosis
c. dehydration

Notice the BUN:Cr ratio is still around 10:1. Therefore, the cause is not renal, so you can eliminate renal failure. I should also add that a common cause of increased BUN and Cr (but still within normal ratio) in an elderly pt (especially one who requires care) is dehydration --> hypovol. So the most likely answer is c. dehydration. If asked for tx? Fluids.

Ref: kaplan path by goljan, renal section

[Anonymous]

you're still helping! Whoever you are, you're awesome. Definitely lots of points with the Man upstairs. Thanks!!!

[Anonymous]

hi step 1,

thanx for still helping other people..may god bless you and you pass with a grest score!
regarding your explanation, i still have a doubt..correct me if i am wrong. as far as i rememeber Dr. Goljan said that if the ratio of BUN and Creatinine is maintained at 10:1, then the cause is renal failiure and if its > 15:1, then its pre renal azotemia..and you have written just the opposite..please explain me..thanx in advance.

good luck..

[Anonymous]

In prerenal azotemia the BUN: Creat ratio is more than 15:1(CMDT 20:1)
whereas renal failure it will be around 10:1 so the case about the lady it is a renal failure not prerenal azo if you look at the BUN: Creat because the ratio is roughly 10:1 it could be the result of antibiotics especially antibiotics like gentamycin the acute tubular necrosis will take a few days to develop.

[Anonymous]

You just illustrate a typical mistake sometimes we understand the concept but we don't apply it correctly on the exam. Step one is rigth about easy concept but US is also rigth in how easy we can make mistakes the BUN 40 Cr 4.2 ratio is 10 - 1 Patient has a renal problem . Agree with US perhaps drugs since elderly have a diminish filtration rate

[nonstopdoc1]

serum BUN:Crt > 20 - the cause is pre-renal

and if its < 20 - the cause is renal..... PERIOD




ok ! atleast most of the times.
cos in medicine 'NEVER' say 'ALWAYS' 8)

[ququ]

In normal individuals, and in patients with intrinsic renal disease, the BUN concentration will be approximately 10 times that of creatinine. Therefore, a BUN/Creatinine ratio considerably greater than 10 suggests a pre﷓renal (i.e. decreased renal perfusion) or post﷓renal (obstruction) cause of renal failure. When the ratio is considerably less than 10, the clinician should suspect liver disease, low protein intake, or ECF volume expansion.

BUN largely depends on the renal flow, creatinine is freely filtered.

[Anonymous]

oops, sorry about that. it looks like you're right about my mixing up the concept. i was trying to write down as many of these questions as I could before i left for on vacation and it seems like i may have rushed through a couple (notice i didn't even put a page reference above which means i didn't check my work). i'm sorry if it caused any confusion :oops:

now that i have more time (just got back into town), i'll write goljan's notes properly and cite his pg ref correctly.


BUN/Creatinine ratio in work-up of oliguria

1. Normal ratio is 10:1

2. prerenal azotemia
A. due to decrease in cardiac output
[list:745fb8005b](1) heart failure
(2) hypovolumia
B. ratio >15:1
(1) clearance of creatinine in urine decreases when glomerular filtration rate decreases
(2) BUN increases disproprtionately due to reabsorption in the proximal tubule
(3) e.g. BUN 80 mg/dL, creatinine 2mg/dL ratio 40:1[/list:u:745fb8005b]

3. acute tubular necrosis
A. ratio remains <15:1
B. both creatinine and BUN are equally affected when there is tubular dysfunction
C. e.g., BUN 80 mg/dL, creatinine 8 mg/dL, ratio=10:1

4. postrenal azotemia
A. due to obstruction of urine flow behind the kidneys
B. initially, ratio is >15:1 but may become <15:1 if obstruction is not relieved and tubular damage occurs


ref: kaplan path by goljan sec 2 pg 144

[Anonymous]

Could you please provide additional details about your reference:
ref: Kap lan path by goljan sec 2 pg 144.

I'd like to purchase this book, but I can't find it on Ama zon.com. I called Kap lan bookstore and they don't seem to have heard about this. Any tips on where I can find this book?

Your assistance is much appreciated! Thanks in advance.

[Anonymous]

looking at a 31/2 year olds blood test with a
bun/cr of 36
bun 18
creat .5

is this a major concern.