Distinguishing between prerenal renal failure (functional renal insufficiency) and acute tubular necrosis (vasomotor nephropathy) is a common problem in clinical medicine which can often be solved by evaluating renal function tests.
Parameters:
(1) urinary sodium concentration
(2) ratio of urine to plasma osmolality
(3) ratio of urine to plasma creatinine
(4) renal failure index (RFI)
(5) fractional excretion (FE) of sodium
(6) ratio of serum BUN to serum creatinine
(7) urine osmolality
Finding
Prerenal Azotemia
Indeterminate
Oliguric Acute Tubular Necrosis
urinary sodium
<20 mEq/L
20-40 mEq/L
> 40 mEq/L
urine-to-plasma osmolality
> 1.5,
rarely < 1.1
1.1-1.5
0.9-1.05,
rarely > 1.5
urine-to-plasma urea
> 8
3-8
< 3
urine-to-plasma creatinine
> 40
20-40
< 20
RFI
<= 1 mEq/L
1.01-2.99 mEq/L
>=3 mEq/L
FE-sodium
< 1%
1%
> 1%
serum BUN-to-creatinine ratio
> 20
10-20
< 10
urine osmolality
> 500
350-500
< 350
Limitations:
• Tests may be affected by diuretics or osmotic diuresis.
• Distinction may be obscure in patients with acute renal disease in a setting of chronic parenchymal renal diseases.
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