Fractional Excretion of Sodium (FENa)
Why Use
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).
When to Use
Patients with oliguria and/or acute kidney injury of unclear etiology.
Formula
Pearls / Pitfalls
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 sodium concentration alone is less accurate because it does not account for urine volume and the kidney’s water handling by antidiuretic hormone (ADH). Single measurements of serum creatinine are “snapshots” in time and do not reflect the true glomerular filtration rate (GFR). The most accurate measurement of GFR is the average of the 24-hr creatinine clearance and urea clearance. The etiology of renal dysfunction cannot be differentiated in those with a FENa <1% who have liver disease.
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.
Advice
No absolute FENa percentage indicates true “prerenal” disease. Always consider the 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).
More Information
Prerenal Indeterminate Intrinsic FENa <1% 1-2% >2% U Na (mmol/L) <20 20-40 >40 Prerenal: 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.