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Old 10-03-2010
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Originally Posted by ashishkabir View Post
If BUN in urine is >15X creatinine in urine then the problem is with the kidney a nephrotic or nephritic syndrome or ATN or ARF or CRF etc...

If BUN is high but Bun / Creatinine is <15 then the problem is volume to the kidney is low for some reason - such as renal artery stenosis or CHF or something else

Is that right?
No, you mixed things up here <a sign that you need a quick FA fix!> . Holding FA right now, specifically on page 469, I can summarize it as shown below:
  • It's serum BUN/Cr, not urine's.
  • A ratio <15 definitely means a renal issue (e.g. ATN). This is because BUN reabsorption is impaired, and so it's lost in urine.
  • A ratio >15 can mean a prerenal or a postrenal problem. To differentiate the two from each other, prerenal problems are usually associated with hypovolemia or decreased RBF, so urine osmolarity is high (i.e. concentrated urine) due to less water lost, not due to excess solute loss (because Na is retained as well).
EDIT: reading your post again, I can see that you're still correct, cause you reversed both the serum and urine ratios and their corresponding causes, but I think it's better to go with the serum ratio, because that's how it will likely show up on the exam.
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ashishkabir (10-04-2010), Sabio (10-03-2010)