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How helpful is fractional sodium excretion?

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How helpful is fractional sodium excretion?

Fractional elimination of sodium (FENa) uses urine chemistry to distinguish between the several causes of acute kidney injury (AKI). Is it a prerenal cause (reduced perfusion), acute tubular necrosis (ATN), or a postrenal cause (obstruction)? Because AKI is commonly multifactorial, we must do not forget that prerenal injury may progress to or coexist with intrinsic renal disease. In the clinical context, FENa is a useful additional data point in patients whose volume status is difficult to evaluate.

Procedure

Collect urine and plasma electrolytes concurrently. Use the next calculation to calculate FENa. (UAlready = sodium in urine, PCr = plasma creatinine, PAlready = plasma sodium, UCr = urine creatinine). There are many online clinical calculators that make it easy to enter patient data to acquire a result.

Interpretation

<1% >1% >4%
Hypovolemia
Heart failure
Renal artery stenosis
Sepsis
Acute tubular necrosis
Interstitial nephritis
Obstacle
– Prostate enlargement
– Kidney stone
– Ureteral obstruction

Clinical considerations

  • Do not use in patients taking diuretics or in patients with chronic kidney disease.
  • FENa percentage mustn’t be considered in isolation from other aspects. The patient’s history, physical examination, clinical context, and current medications should at all times be taken under consideration.
  • Non-volume depleting conditions which will cause low or borderline FENa include: acute glomerulonephritis, contrast-induced nephropathy, cardiorenal syndrome, and hepatorenal syndrome.

Espinel CH (1976). The FENa test. Application within the differential diagnosis of acute renal failure. , (6), 579–581. https://doi.org/10.1001/jama.236.6.579

Fractional sodium excretion (FENa). MDCalc. https://www.mdcalc.com/calc/60/fractional-excretion-sodium-fena

Okusa, M. and Chopra T. (2022, November 15). Fractional elimination of sodium, urea and other molecules in acute kidney injury. https://www.uptodate.com/contents/frakcyjne-wydalanie-of-sodu-mocznika-i-inne-molecules-in-acute-kidney-injury

Pahwa, AK and Sperati, CJ (2016). Urinary excretion fraction indices within the assessment of acute kidney injury. , (1), 77–80. https://doi.org/10.1002/jhm.2501

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