Renal Failure and BUN/Cr ratio

[MudPhud20XX]

Alright so both BUN and Cr are by products of our body and in general if they are increased, it could indicate kidney problem among many.

Can anyone explain why we make such a big deal out of BUN/Cr ratio?

Also, I can't seem to grasp how you can differentiate prerenal, intrarenal, and postrenal by looking at BUN/Cr.

For example, here is what Kaplan physio says:

If BUN/Cr > 15, then it's either prerenal or postrenal failure (Increased fractional excretion of Na+ separates the two, if it's increased then it's postrenal)

If BUN/Cr < 15, then it's intrarenal

So can anyone walk me through these concepts of BUN/Cr and also explain why increased fractional excretion of Na+ indicates postrenal?

Many thanks in advance.

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[Nephronlearner]

Pre-renal:
Low cardiac output -> low pressure -> low filtration. At the same time it increases reabsorption (driven by pressure in peritubular capillaries, which is again low due to decreased CO). Creatinine, however, is not reabsorbed. Thereby you get an increase in BUN -> increased ratio.
The function of the tubule remains fine, so sodium excretion will remain normal (<1% of what was filtered).

Post-renal happens due to obstruction. Long term obstruction leads to tubular epithelium damage. Bad epithelium can't reabsorb as much sodium.

 

[swoopyswoop]

watch the pathoma section on this. he explains it very well.