Fractional Excretion of Sodium (FENa)
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.
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).
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- 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).
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.
Fractional Excretion of Sodium (FENa), % = 100 × (SCr × UNa ) / (SNa × UCr)
SCr, serum creatinine; UNa, urine sodium; SNa, serum sodium; UCr, urine creatinine.
How the equation is derived:
- FENa is a measure of tubular resorption of Na.
- FENa = (Na excreted/Na filtered) x 100.
- Na excreted = UNa × urine volume.
- Therefore, Na filtered = PNa × (UCr × urine volume)/PCr.
Facts & Figures
Pre-Renal: Anything causing decreased effective renal perfusion: hypovolemia, heart failure, renal artery stenosis, sepsis, etc. Remember, contrast-induced nephropathy will often look pre-renal.
Intrinsic: ATN, AIN, glomerulonephritides, etc.
Post-Renal: Obstruction (e.g. BPH, bladder stone, bilateral ureteral obstruction).
Espinel in 1976 originally described a FENa cutoff of <1% for pre-renal azotemia in a study study of 17 adults with oliguria, defined as urine output <20mL/hr. Patients with urinary obstruction, acute glomerulonephritis, CKD, and diuretic use were excluded.
A FENa cutoff of <1% for pre-renal azotemia was clinically validated in a prospective study, though there was overlap between patients with true pre-renal azotemia and acute tubular necrosis. Specificity for pre-renal azotemia (with a cutoff of <1%) was decreased in non-oliguric acute renal failure (Miller 1978).
At a normal GFR of 180 L/day and Na concentration of 140 mEq/L, the filtered sodium load is 25,200 mEq/day (=140 × 180). A FENa of 1% in this setting represents sodium excretion of 252 mEq/day. This is higher than the average sodium intake of 80 to 250 mEq/day. This is the physiologic basis for hypothesizing that patients with normal GFR have a FENa below 1%.
Original/Primary ReferenceEspinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-81.
ValidationMiller TR, Henrich WH, Schrier RW. Urinary diagnostic indices in acute renal failure. Ann Int Med. 1978; 89:47-50
About the Creator
Carlos Espinel, MD, is a cardiologist in Arlington, Virginia and is affiliated with Virginia Hospital Center. In addition to developing the FENa test, his research is the scientific basis for the US Food Labeling Law, which stipulates that food labels must list the exact amount of sodium and other contents. Dr. Espinel is also the creator of ArtMedicine, a medical education model integrating science, humanities and the arts.
To view Dr. Carlos Espinel's publications, visit PubMed
- Devika Nair, MD