Curated insights • How it Works • Practical Pearls • Evidence Base
Requires simultaneous (paired) blood and spot urine samples for Sodium and Creatinine.
FENa measures the percentage of sodium filtered by the kidney that is actually excreted in the urine. In prerenal states, the kidneys attempt to maximize volume by reabsorbing sodium. In ATN, tubular damage prevents efficient reabsorption, leading to "salt wasting."
| < 1% |
| 1% – 2% |
| > 2% |
Loop diuretics inhibit sodium reabsorption in the thick ascending limb, artificially elevating FENa even in prerenal states. Use FEUrea (Fractional Excretion of Urea) instead if the patient has received diuretics within 24 hours.
FENa is less reliable in patients with pre-existing CKD, as damaged tubules in a chronic state may have a baseline defect in sodium handling regardless of the acute insult.
The FENa test. Use in the differential diagnosis of acute renal failure.
Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure.
Before the FENa, clinicians relied on simple urine sodium concentration. However, urine sodium is highly dependent on urine volume. Dr. Espinel's introduction of "fractional excretion" normalized sodium clearance to the GFR (using creatinine as a proxy), providing a far more accurate reflection of tubular function.
Sodium (mEq/L)
Creatinine (mg/dL)
Sodium (mEq/L)
Creatinine (mg/dL)
Curated insights • How it Works • Practical Pearls • Evidence Base
Requires simultaneous (paired) blood and spot urine samples for Sodium and Creatinine.
FENa measures the percentage of sodium filtered by the kidney that is actually excreted in the urine. In prerenal states, the kidneys attempt to maximize volume by reabsorbing sodium. In ATN, tubular damage prevents efficient reabsorption, leading to "salt wasting."
| < 1% |
| 1% – 2% |
| > 2% |
Loop diuretics inhibit sodium reabsorption in the thick ascending limb, artificially elevating FENa even in prerenal states. Use FEUrea (Fractional Excretion of Urea) instead if the patient has received diuretics within 24 hours.
FENa is less reliable in patients with pre-existing CKD, as damaged tubules in a chronic state may have a baseline defect in sodium handling regardless of the acute insult.
The FENa test. Use in the differential diagnosis of acute renal failure.
Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure.
Before the FENa, clinicians relied on simple urine sodium concentration. However, urine sodium is highly dependent on urine volume. Dr. Espinel's introduction of "fractional excretion" normalized sodium clearance to the GFR (using creatinine as a proxy), providing a far more accurate reflection of tubular function.
