Global Health
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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