ACUTE REAL FAILURE (ARF).[1]

Acute renal failure is defined as an abrupt decline in renal function with increased creatinine and blood urea nitrogen levels (BUN) over a period of several days. There are three main categories of ARF:

1)    Pre-renal azotemia :decreased renal perfusion causes a decreased glomerular filtration rate. In response, the kidney acts to retain salt and water in an attempt to increase blood pressure and perfusion.

2)    Intrinsic renal failure: generally due to acute tubular necrosis, which leads to obstruction of renal tubules with necrotic debris and subsequent fluid backflow with decreased GFR. In this setting the damages renal tubules are unable to retain salt and water.

3)    Post-renal azotemia: caused by outflow obstruction and can only occurs when such obstruction is bilateral. In this setting there is also backflow to fluid with decreased GFR.

Variable	Prerenal	Renal	Postrenal
Urine osmolality	>500	<350	<350
Urine Na	<10	>20	>40
FE Na	<1%	>2%	>4%
BUN/Cr Ratio	>20	<15	>15

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[1] (Le, Bhushan, Tolles, & Hofmann, 2011)