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Interpreting the fractional excretion of sodium

  • Robert W. Steiner
    Correspondence
    Requests for reprints should be addressed to Dr. Robert W. Steiner, H 781, University of California at San Diego Medical Center, 225 Dickinson Street, San Diego, California 92103.
    Footnotes
    Affiliations
    San Diego, California USA
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  • Author Footnotes
    1 From the Department of Medicine, University of California at San Diego School of Medicine, and the Rees Stealy Medical Group, San Diego, California.
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      Abstract

      In most normal subjects, the fractional excretion of sodium is usually less than 1 percent but may be raised with an increase in salt intake. In acutely azotemic patients, a low fractional excretion of sodium usually indicates a prerenal process that is responsive to volume repletion. However, such a low fractional excretion of sodium also can be seen with azotemia due to hepatic or cardiac failure, as well as acute glomerulonephritis, pigment nephropathy, contrast nephrotoxicity, polyuric renal failure associated with burns, acute obstruction, renal transplant rejection, and occasionally non-oliguric acute renal failure, none of which is a volume-responsive process. A fractional excretion greater than 1 percent in acutely azotemic patients usually Indicates intrinsic renal injury, but is consistent with volume depletion in patients receiving diuretics or in some patients with chronic renal insufficiency. Similarly, a low quotient in acute renal parenchymal injury is usually interpreted to indicate wide-spread tubular integrity, but is consistent with several different pathophysiotogic processes. The fractional excretion of sodium must be interpreted in light of the specific clinical setting and other laboratory data to be useful in patient management.
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