Calc Function

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    Fractional Excretion of Sodium (FENa)

    Determines if renal failure is due to pre-renal, intrinsic, or post-renal pathology.
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    INSTRUCTIONS

    Do not use in patients taking diuretics or with known chronic kidney disease, urinary tract obstruction, or acute glomerular disease. Use FEUrea in patients on diuretics.

    When to Use
    Pearls/Pitfalls
    Why Use

    Patients with oliguria and/or acute kidney injury of unclear etiology.

    • FENa is only clinically validated in patients with oliguric acute kidney injury WITHOUT any of the following: diuretic use, chronic kidney disease (CKD), urinary tract obstruction, or acute glomerular disease.
    • Using urine Na concentration alone is less accurate because it does not account for urine volume and water handling of the kidney by antidiuretic hormone (ADH).
    • Single measurements of serum creatinine are “snapshots” in time and do not reflect true glomerular filtration rate (GFR). The most accurate measurement of GFR is the average of the sum of 24h creatinine clearance and urea clearance.
    • May give an additional data point in patients whose volume status is difficult to assess.
    • Provides a more accurate assessment of kidney function than urine sodium alone (for example, a severely hypovolemic patient may have a relatively high urine sodium, as a fraction of total urine volume, despite having little sodium in the urine).
    mEq/L
    mg/dL
    mEq/L
    mg/dL

    Result:

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    Next Steps
    Evidence
    Creator Insights

    Advice

    • No FENa percentage will always accurately suggest “pre-renal” disease. Always consider history, clinical context, physical exam, and current medications.
    • Obtaining repeat FENa or urine studies throughout a patient’s hospital course can give more clinical clues.
    • Non-volume depleted states with low urine sodium (and consequently low FENa) include: acute glomerulonephritis, cardiorenal syndrome, hepatorenal syndrome, contrast-related nephropathy, and rarely, acute obstruction and early acute interstitial nephritis (AIN) or acute tubular necrosis (ATN).

    Management

    In patients with acute kidney and suspected obstructive uropathy, consider bedside ultrasound to evaluate for hydronephrosis, bladder distention, etc. Empiric Foley catheter placement may also be valid in some settings, though it may also be associated with complications, and does not rule out ureteral obstructions above the bladder.

    Content Contributors
    • Devika Nair, MD
    About the Creator
    Dr. Carlos Espinel
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    Content Contributors
    • Devika Nair, MD