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Biomarkers of Acute Renal Failure



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Slide 14

August 2009

In addition to the urinalysis, urine electrolytes can also be helpful for differentiating prerenal causes of decreased renal function from true acute kidney injury. If a patient has oliguria or decreased urine output due to a prerenal cause, one would expect the kidney to avidly reabsorb sodium and chloride. Therefore, the concentration of urinary sodium or chloride should be reduced to <10 mEq/L. This can be normalized to creatinine by calculating a fractional excretion of sodium or chloride. Thus, a fractional excretion of sodium <1% would be consistent with a prerenal cause. Looked at another way, if a person’s kidney is able to reabsorb sodium effectively and achieve a fractional excretion of sodium <1%, this is a sign that true cellular injury or ATN has not yet occurred. Many patients in the hospital have received diuretics, which would tend to increase urinary sodium and chloride excretion. In this circumstance, calculating a fractional excretion of urea may also be useful, since urea handling by the kidney is less influenced by diuretics than sodium and chloride. A fractional excretion of urea <35% is more consistent with prerenal causes of decreased kidney function.

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