GOLJAN-- prerenal and renal azotemia

[MDpride]

Hii

RR says that extrarenal loss of urea takes place causing BUN/CREATININE ratio to be < 15 before entering kidney in RENAL AZOTEMIA.

Why is extrarenal loss of urea not happening in PRERENAL AZOTEMIA?

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[Miss Alyssa]

RR says that prerenal azotemia is caused by a decreased CO --> decreased GRF --> back up of urea and creatinine in the blood. So both urea and creatinine are increased. Of the urea and creatinine that get filtered, creatinine cannot be reabsorbed but urea can. The decreased flow allows more urea to be reabsorbed further increasing the urea in the blood (and increasing the urea:creatinine)

 

[MDpride]

RR says that prerenal azotemia is caused by a decreased CO --> decreased GRF --> back up of urea and creatinine in the blood. So both urea and creatinine are increased. Of the urea and creatinine that get filtered, creatinine cannot be reabsorbed but urea can. The decreased flow allows more urea to be reabsorbed further increasing the urea in the blood (and increasing the urea:creatinine)

because there is back up of urea and creatinine,there is more time for extrarenal urea loss?

so there should be some factor that determines when extrarenal loss of urea will take place?

 

[Sora]

There are factors that regulate extrarenal WATER loss to conserve fluid in contracted states, which will therefore affect extrarenal UREA loss (remember urea is amphiphilic), including ADH. When you get a back-up of urea and creatinine in PRErenal azotemia, you are volume contracted and will therefore have decreased extrarenal loss of water (and urea). In renal azotemia, you are volume expanded, and will get increased extrarenal water loss, and therefore increased extrarenal urea loss.

However, I wouldn't worry about any of this for the boards. It does seem like anything that would be emphasized. I think the key is that renal azotemia means BUN:creatinine =< 15.

 

[Rabbit Hole]

Hii

RR says that extrarenal loss of urea takes place causing BUN/CREATININE ratio to be < 15 before entering kidney in RENAL AZOTEMIA.

Why is extrarenal loss of urea not happening in PRERENAL AZOTEMIA?

BUN:Creatinine is a ratio.
Normal BUN = 10
Normal Cr = 1
Normal BUN:Cr = 10:1

Prerenal azotemia BUN:Cr approximately 15:1
- Only BUN level is elevated
- You got low blood flow and have extra time to pick up some urea.

Renal failure BUN:Cr approximately 10:1
- Both BUN and Cr levels are elevated
- You got a problem with your glomeruli so both urea and creatinine get elevated.


.. or something like that.
(Goljan talks about this in his audio)

 

[rem6775]

BUN:Creatinine is a ratio.
Normal BUN = 10
Normal Cr = 1
Normal BUN:Cr = 10:1

Prerenal azotemia BUN:Cr approximately 15:1
- Only BUN level is elevated
- You got low blood flow and have extra time to pick up some urea.

Renal failure BUN:Cr approximately 10:1
- Both BUN and Cr levels are elevated
- You got a problem with your glomeruli so both urea and creatinine get elevated.


.. or something like that.
(Goljan talks about this in his audio)

That part is not true. In prerenal azotemia both Cr and BUN are increased, but BUN is increased more so than Cr because it is reabsorbed by the kidney (and Cr cannot be reabsorbed). They are both elevated, however.

 

[MDpride]

There are factors that regulate extrarenal WATER loss to conserve fluid in contracted states, which will therefore affect extrarenal UREA loss (remember urea is amphiphilic), including ADH. When you get a back-up of urea and creatinine in PRErenal azotemia, you are volume contracted and will therefore have decreased extrarenal loss of water (and urea). In renal azotemia, you are volume expanded, and will get increased extrarenal water loss, and therefore increased extrarenal urea loss.

However, I wouldn't worry about any of this for the boards. It does seem like anything that would be emphasized. I think the key is that renal azotemia means BUN:creatinine =< 15.

THANK YOU
Volume Contraction didn't come to mind. thats a logical explanation.

 

[Super Mario]

1) Pre-renal failure:
kidney senses decreased blood flow --> normal response is increased proximal water and sodium reabsorption --> urea passively follows H2O and Na --> therefore increased BUN relative to creatinine (ratio > 20) and fractional excretion of Na < 1% highly suggestive of prerenal failure

Caveat:Malnutrition (low protein intake) or chronic liver disease (low protein production) --> low urea --> BUN/creatinine may not be elevated --> BUN/creatinine <20 does not exclude prerenal failure

2) Intrinsic renal disease (ATN, AIN, glomerular disease): kidney not functioning properly --> unable to reabsorb H2O and Na properly --> since H2O and Na not being reabsorbed, urea does not follow passively --> BUN/creatinine < 20, FeNa > 1%

Hope that helps