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The Fractional Excretion of Sodium (FENa) is a calculated index that expresses the percentage of filtered sodium that is ultimately excreted in the urine. It is used primarily to distinguish pre-renal acute kidney injury (AKI) from intrinsic renal causes such as acute tubular necrosis (ATN). In pre-renal states — where kidney perfusion is reduced due to dehydration, heart failure, sepsis, or haemorrhage — the tubules remain intact and avidly reabsorb sodium to restore intravascular volume, resulting in a FENa below 1%. In ATN, tubular cells are injured and lose their ability to reabsorb sodium efficiently, producing a FENa above 2%. Values between 1% and 2% fall in an indeterminate zone. However, FENa has important limitations: it is unreliable in patients who have received diuretics (which increase sodium excretion regardless of tubular function), in those with CKD (where baseline tubular handling is altered), in contrast nephropathy, myoglobinuric renal failure, and in some forms of early ATN. In diuretic-treated patients, the Fractional Excretion of Urea (FEUrea) is a superior alternative. Despite its limitations, FENa remains a valuable rapid bedside calculation in the workup of new oliguria or anuria. Acute kidney injury (AKI) affects up to 20% of hospitalised patients and is associated with significantly increased short- and long-term mortality. One of the most critical early decisions in AKI management is distinguishing pre-renal azotaemia — where the kidney retains normal tubular function and is responding appropriately to reduced perfusion — from intrinsic renal injury such as acute tubular necrosis (ATN), where tubular function is impaired. FENa provides a physiologically rational approach to this distinction by measuring how much of the filtered sodium is actually excreted, reflecting tubular sodium avidity. In pre-renal states, intact tubules aggressively reabsorb sodium (FENa < 1%), while in ATN, damaged tubules cannot reabsorb sodium effectively (FENa > 2%). This calculation is most valuable in oliguric AKI and should be performed before giving IV fluids or diuretics.
FENa (%) = (UNa × PCr) / (PNa × UCr) × 100 Where each variable represents a specific measurable quantity in the finance and lending domain. Substitute known values and solve for the unknown. For multi-step calculations, evaluate inner expressions first, then combine results using the standard order of operations.
- 1Collect simultaneous spot urine and blood samples: measure urine sodium (UNa, mEq/L), plasma sodium (PNa, mEq/L), urine creatinine (UCr, mg/dL), and plasma creatinine (PCr, mg/dL).
- 2Calculate the numerator: urine sodium × plasma creatinine (UNa × PCr). This represents the fraction of sodium appearing in the urine relative to the amount filtered.
- 3Calculate the denominator: plasma sodium × urine creatinine (PNa × UCr). This normalises for the glomerular filtration of sodium.
- 4Divide numerator by denominator and multiply by 100 to express as a percentage.
- 5Interpret: FENa <1% indicates avid tubular sodium reabsorption, consistent with pre-renal physiology. FENa >2% indicates impaired tubular sodium reabsorption, consistent with ATN.
- 6Apply clinical context: always consider volume status, diuretic use, underlying CKD, and clinical history before interpreting the result.
- 7If diuretics have been given within 12–24 hours, use FEUrea (normal <35% in pre-renal; >50% in ATN) instead.
FENa <1% confirms pre-renal physiology — expect rapid response to IV fluids
(12 × 2.8) / (142 × 180) × 100 = 33.6 / 25560 × 100 = 0.13%. Kidneys are avidly reabsorbing sodium, indicating intact tubular function with reduced perfusion.
FENa >2% consistent with established ATN — IV fluids alone will not restore kidney function
(68 × 3.5) / (138 × 55) × 100 = 238 / 7590 × 100 = 3.1%. Injured tubular cells cannot reabsorb sodium normally, leading to sodium wasting in urine despite systemic fluid depletion.
Furosemide blocks tubular sodium reabsorption pharmacologically, making FENa >1% even in pre-renal states — use FEUrea instead
Loop diuretics directly inhibit the Na-K-2Cl cotransporter in the thick ascending limb of Henle, forcing sodium into the urine regardless of perfusion status. FEUrea should be calculated: if FEUrea <35%, pre-renal is more likely.
Contrast nephropathy frequently produces FENa <1% even when intrinsic injury is present — FENa is unreliable here
Contrast-induced nephropathy may cause intense renal vasoconstriction with preserved tubular sodium reabsorption early on, mimicking pre-renal physiology despite direct tubular toxicity. Clinical context is essential.
Mortgage lenders and loan officers use Fractional Excretion Sodium to structure repayment schedules, compare fixed versus adjustable rate options, and calculate total borrowing costs for residential and commercial real estate transactions across different term lengths.
Personal finance advisors apply Fractional Excretion Sodium when counseling clients on debt reduction strategies, comparing the mathematical benefit of accelerated payments against alternative investment returns to determine the optimal allocation of surplus cash flow.
Credit unions and community banks rely on Fractional Excretion Sodium to generate accurate Truth in Lending disclosures, ensure regulatory compliance with TILA and RESPA requirements, and provide borrowers with standardized cost comparisons across competing loan products.
Corporate treasury departments use Fractional Excretion Sodium to model the cost of revolving credit facilities, term loans, and commercial paper programs, optimizing the company's capital structure and minimizing weighted average cost of debt financing.
Zero or negative interest rate
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in fractional excretion sodium calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Balloon payment at maturity
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in fractional excretion sodium calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Variable rate mid-term adjustment
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in fractional excretion sodium calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Acute urinary obstruction (post-renal AKI)
In acute bilateral ureteric obstruction, FENa is often <1% early, because the back-pressure does not immediately disrupt tubular sodium transport. FENa is therefore not useful for distinguishing post-renal from pre-renal AKI. Bladder ultrasound and renal tract imaging are essential for ruling out obstruction in any patient presenting with oliguria or anuria.
| FENa | Interpretation | Typical Causes | Caveat |
|---|---|---|---|
| < 1% | Pre-renal AKI | Dehydration, heart failure, cirrhosis, sepsis | May also be <1% in contrast nephropathy, myoglobinuria, early ATN |
| 1–2% | Indeterminate | Overlap zone | Clinical context essential; consider FEUrea |
| > 2% | Intrinsic renal (ATN) | Ischaemia, nephrotoxins, sepsis-ATN | May be elevated by diuretics — check medication list |
| Any value (on diuretics) | Unreliable | Loop or thiazide diuretics within 24h | Use FEUrea instead |
| < 35% (FEUrea) | Pre-renal (diuretic-treated) | Pre-renal physiology despite diuretics | Preferred test when diuretics used |
What does FENa <1% mean?
FENa <1% indicates that the kidneys are retaining more than 99% of filtered sodium, reflecting intact and activated tubular reabsorption. This occurs when there is reduced renal perfusion (pre-renal state) from any cause — hypovolaemia, congestive heart failure, cirrhosis, or sepsis-induced hypotension. The tubules are working hard to conserve volume, which means the kidney parenchyma itself is not the primary problem.
Why is FENa unreliable in patients on diuretics?
Loop diuretics (furosemide, bumetanide) and thiazides pharmacologically block tubular sodium reabsorption, elevating urinary sodium and FENa regardless of the underlying aetiology of AKI. A patient with pre-renal AKI who received a single dose of furosemide may have a FENa of 3–4%, falsely suggesting ATN. In this scenario, FEUrea is preferred because urea reabsorption is less affected by diuretics.
What is the difference between FENa and FEUrea?
FEUrea (fractional excretion of urea) uses urea nitrogen instead of sodium: FEUrea = (U-urea × P-creatinine) / (P-urea × U-creatinine) × 100. FEUrea <35% suggests pre-renal AKI; >50% suggests ATN. It is less affected by diuretics because diuretics do not block urea reabsorption. FEUrea is the preferred test in diuretic-treated patients, though it is slightly less sensitive and specific than FENa in diuretic-naive patients.
Can FENa be used in CKD?
FENa has reduced diagnostic accuracy in patients with pre-existing CKD, because CKD kidneys have impaired baseline sodium reabsorption. Patients with CKD may have a FENa above 1% at baseline even without ATN. Any new AKI in CKD must be interpreted against the patient's known baseline creatinine and prior urine sodium data, and FENa thresholds should be applied with caution.
Is FENa useful in non-oliguric AKI?
FENa is most informative in oliguric AKI (<400 mL/day). In non-oliguric AKI — where urine output is preserved despite rising creatinine — FENa has lower discriminatory value and the thresholds of 1% and 2% are less reliable. Non-oliguric ATN, common after aminoglycoside toxicity or contrast, may present with FENa values that overlap with the pre-renal range.
What is the normal FENa in a healthy person?
In a healthy individual on a normal sodium diet, FENa is typically 1–2%, as the kidneys excrete dietary sodium to maintain balance. The clinically important distinction arises in the context of AKI: a FENa of 0.2% in a patient with acute oliguria strongly suggests pre-renal physiology, while the same value in a healthy person is normal dietary sodium excretion.
Does FENa work in rhabdomyolysis-induced AKI?
Myoglobinuric AKI (from rhabdomyolysis) is a notable exception where FENa may be <1% despite intrinsic tubular injury, because myoglobin-induced renal vasoconstriction and cast formation can preserve some sodium reabsorption early in the course. This can falsely suggest pre-renal aetiology. Clinical context (markedly elevated CK, cola-coloured urine, elevated urine myoglobin) is essential.
How quickly can FENa change with fluid resuscitation?
In true pre-renal AKI, FENa may normalise within 1–2 hours of adequate fluid resuscitation as renal perfusion is restored and tubular stress is relieved. Failure to see a clinical response (improving urine output and creatinine) despite adequate fluids, combined with a persistently elevated FENa, supports the transition from pre-renal to established ATN.
Consejo Pro
In a patient on furosemide, calculate FEUrea as your primary discriminator: FEUrea = (U-Urea × P-Cr) / (P-Urea × U-Cr) × 100. A result <35% points to pre-renal AKI; >50% to established ATN. Unlike sodium, urea reabsorption is not directly blocked by loop diuretics, making FEUrea more reliable in this common clinical scenario.
¿Sabías que?
The concept of fractional excretion was first applied clinically in the early 1970s. Espinel introduced FENa in 1976 as a simple bedside test to guide fluid management in AKI — the same year Cockcroft-Gault published their creatinine clearance equation — making 1976 a remarkably productive year for practical nephrology tools.
Referencias
- ›Espinel CH — The FENa Test (JAMA 1976)
- ›KDIGO 2012 Clinical Practice Guideline for Acute Kidney Injury
- ›Pahwa AK et al. — FEUrea vs FENa in diuretic-treated AKI (Clin J Am Soc Nephrol 2016)
- ›UpToDate — Diagnostic approach to the patient with acute kidney injury
- ›Schrier RW — Renal and Electrolyte Disorders, 8th ed.