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Old 10-30-2011
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Kidney Acute Renal Failure-Step 1

I'm really confused with the lab results table regarding prerenal, renal and postrenal failure(FA 2011,page 471). Is anybody here willing to give me some clues?

Appreciate your reply!
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Old 10-30-2011
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Default okay let me see what i understand

this might be a little lengthy, all of this is from goljan pathology.

(Cr cannot be absorbed in the kidneys, Urea can be lost form sites other than the kidneys.)

in pre-renal failure, which is mostly due to hypovolemia, the GFR is reduced as such urea and Cr back up into the blood and their conc rise. in addition, the kidneys reabsorb urea and water and Na form the tubular fluid in response to the hypovolemia inorder to raise the blood volume back to normal. As such urine osmolality is high, sodium is low hence the low FeNa which i believe is urine NA divided by serum Na.

in renal azotemia, theres also reduced GFR so urea and Cr back up into the blood HOWEVER theres extra renal loss of urea in sites such as the skin, so the serum BUN/Cr ratio is less than 15. much of the proximal tubule is sloughed off so urea is lost in urine. since much of the Na reabsorbed normally is form the proximal tubule which are sloughed off in this case, then the urine Na is greater than for pre-renal and the FeNa is also greater. remember that 66% of water is reabsorbed i the proximal tubule, so a lot of water is lost hence the lower urine osmolality.

post renal failure causes back up of tubular fluid, it initiates the tubulo-glomerular feed back, so GFR reduces hence theres a proportinate increase in urea and Cr. howver the pressure bluid up in the tubules causes a back-diffusion of urea not Cr so that the ratio of Urea/Cr is greater than for renal azotemia (but less than for pre-reanl azotemia where urea is actively reabsored form the PCT)

i cant explain why the urine Na is much higher in post-renal than the others. i am also worried that instead of making it better i have confused you the more! hopefully some one who really gets it will respond to the post.
i think the exact digits e.g 20, 16, 2% etc have to be memorized but if one understands the mechanism it should make memorization easier.
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The above post was thanked by:
belindalimm (10-30-2011), dr.muhamad (10-31-2011)


Pathology-, Renal-

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