Renal versus prerenal failure

The following investigations may be used to distinguish renal and prerenal failure:

• urine osmolarity, more than 500 mOsm in prerenal, less than 350 in renal failure
• urinary sodium, less than 20 in prerenal, around 40 in renal
• urine to plasma urea ratio, more than 8 in prerenal, less than 3 in renal failure
• urine to plasma creatinine ratio, more than 40 in prerenal, less than 20 in renal failure
• fractional sodium excretion, less than 1% in prerenal, greater than 2% in renal failure

These differences are a little easier to remember if it is remembered that in prerenal failure the changes are normal physiology. Note that these values are of limited clinical value because often intermediate values are encountered; also these indices are influenced by other factors such pre-existing tubular disease and diuretics