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Renal Failure Lab Tests

Acute renal failure (ARF) is a sudden loss of renal function occurring over several hours to days. An increased serum creatinine concentration, accumulation of nitrogenous wastes and a decline in urinary output are the hallmarks of ARF. An increase of serum creatinine level by more than 0.5 mg/dL, a 50% increase of serum creatinine over a baseline level, or a >25% decline in creatinine clearance are accepted indicators of ARF. Oliguria is defined as a urinary output of <400 mL per day. Anuria is defined as <50 mL per day.

ARF occurs in approximately 1% of hospitalized patients and in approximately 20% of patients treated in ICUs. Community acquired ARF occurs in approximately 200 people per 1 million population.

More than 50 causes of acute renal failure have been identified. Traditionally, the evaluation of acute renal failure has focused on the determination of whether the cause of renal failure is prerenal due to decreased perfusion, postrenal due to obstruction of urinary outflow, or intrinsic remal due to damage to the renal tubules, glomeruli or interstitium.

The various serum and urinary laboratory findings used in classifying acute renal failure are summarized in the following table.



Intrinsic Renal


BUN/Cr ratio


10 – 20

10 – 20

Urine specific gravity



>1.010 early

<1.010 late

Urine osmolality



>400 early

~300 late

Urine sodium



<20 early

>40 late



>2 – 3

<1 early

>3 late

Urine Cr/Plasma Cr



>40 early

<=20 late

Urine microscopy




A low fractional excretion of sodium (<1%) suggests that oliguria is likely due to decreased renal perfusion. The kidney responds appropriately by decreasing the excretion of filtered sodium to improve plasma volume and renal perfusion.

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