What does FENa tell you?

FENa (fractional excretion of sodium) measures what percentage of sodium filtered by the kidneys is excreted in the urine. A low FENa ( 2%) suggests the tubules are damaged and cannot reabsorb sodium, consistent with intrinsic renal disease such as acute tubular necrosis (ATN).

What is a normal FENa value?

In a healthy person, FENa is typically 1–3% under normal conditions, as the kidneys regulate sodium balance. In the context of acute kidney injury: FENa 2% suggests intrinsic renal disease. These thresholds assume the patient is not on diuretics.

When should I use FEUrea instead of FENa?

FEUrea (fractional excretion of urea) should be used when a patient is on diuretics (furosemide, thiazides, etc.). Diuretics increase urinary sodium excretion, which falsely elevates FENa even in prerenal states. FEUrea 50% suggests intrinsic renal disease. FEUrea is less affected by diuretics because urea handling is independent of the sodium-potassium pumps that diuretics block.

Can FENa be low in ATN?

Yes, FENa can be <1% in some forms of intrinsic renal disease, including contrast-induced nephropathy, myoglobinuria (rhabdomyolysis), hemoglobinuria, acute glomerulonephritis, and early obstructive uropathy. This is why FENa must always be interpreted in clinical context and not used as a standalone diagnostic tool.

What are the most common causes of prerenal azotemia?

Prerenal azotemia (FENa <1%) results from reduced kidney perfusion without intrinsic damage. Common causes include volume depletion (vomiting, diarrhea, hemorrhage, burns), decreased cardiac output (heart failure, cardiogenic shock), hepatorenal syndrome in liver failure, NSAIDs (which constrict the afferent arteriole), and ACE inhibitors/ARBs (which dilate the efferent arteriole). Early treatment of the underlying cause can reverse the injury before it becomes structural.

How do I interpret a FENa of 1.5%?

A FENa of 1–2% falls in the indeterminate range and overlaps both prerenal and intrinsic causes. The clinical picture must guide interpretation. Consider the patient's volume status, urine output, urine microscopy (casts suggest ATN), medication history (diuretics invalidate FENa), and trend of creatinine. If the patient is on diuretics, calculate FEUrea instead. Serial measurements and clinical correlation are essential in the indeterminate range.

What are the KDIGO AKI staging criteria?

KDIGO (2012) defines AKI stages by creatinine rise or urine output: Stage 1 — creatinine ×1.5–1.9 times baseline OR increase ≥0.3 mg/dL within 48 hours, OR urine output <0.5 mL/kg/hr for 6–12 hours. Stage 2 — creatinine ×2.0–2.9 times baseline OR urine output <0.5 mL/kg/hr for ≥12 hours. Stage 3 — creatinine ×3.0 times baseline, OR creatinine ≥4.0 mg/dL, OR initiation of RRT, OR urine output <0.3 mL/kg/hr for ≥24 hours. FENa helps determine AKI cause but KDIGO staging quantifies its severity.

What is hepatorenal syndrome and how does FENa help diagnose it?

Hepatorenal syndrome (HRS) is a severe form of functional renal failure in patients with advanced cirrhosis and portal hypertension. Despite profound kidney impairment, the tubules remain structurally intact — so the kidneys avidly retain sodium, resulting in FENa typically <1% (often <0.1%). This low FENa helps distinguish HRS from ATN in cirrhotic patients. HRS is a diagnosis of exclusion requiring absence of volume depletion, nephrotoxic drugs, shock, and structural kidney disease.

FENa Calculator — Fractional Excretion of Sodium

Calculates fractional excretion of sodium to differentiate prerenal azotemia from intrinsic renal disease.

What Is FENa and Why Does It Matter?

This FENa calculator computes fractional excretion of sodium from serum and urine sodium and creatinine values, then interprets whether the result suggests prerenal azotemia or intrinsic renal disease. FENa is one of the most important tools for evaluating acute kidney injury (AKI) — when kidneys are responding to low perfusion, FENa falls below 1%; when tubular cells are damaged (ATN), FENa rises above 2%.

The formula is: FENa (%) = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100

How to Interpret FENa Results

The classic thresholds for FENa interpretation are:

FENa <1%: Prerenal — kidney is intact but underperfused. Causes: volume depletion, heart failure, hepatorenal syndrome, NSAID use.
FENa 1–2%: Indeterminate — cannot reliably distinguish prerenal from ATN. Use clinical context.
FENa >2%: Intrinsic renal disease — tubular damage. Causes: ATN (ischemic or nephrotoxic), glomerulonephritis, acute interstitial nephritis.

These thresholds only apply to patients who are NOT on diuretics. For patients on loop or thiazide diuretics, use the FEUrea tab instead. You may also want to check your GFR to quantify the degree of renal impairment.

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When FENa Is Unreliable

FENa has several important false-positive and false-negative scenarios:

Diuretics: Loop and thiazide diuretics increase urinary sodium, falsely elevating FENa even in prerenal states. Always use FEUrea in these patients.
Low FENa in ATN: Contrast nephropathy, myoglobinuria, hemoglobinuria, and early obstructive uropathy can all cause ATN with FENa <1%.
Chronic kidney disease: Baseline FENa is higher in CKD patients, lowering the specificity of the 1% threshold.
Non-oliguric ATN: Some ATN cases present with normal urine output and may have FENa in the indeterminate range.

FEUrea: The Alternative for Diuretic Patients

For patients on diuretics, FEUrea (fractional excretion of urea) is the preferred test. Unlike sodium, urea handling is less affected by diuretics:

FEUrea (%) = (Urine Urea × Serum Cr) / (Serum Urea × Urine Cr) × 100

FEUrea <35%: Prerenal — kidney is intact, avid urea reabsorption
FEUrea >50%: Intrinsic renal disease — ATN or structural damage
FEUrea 35–50%: Indeterminate

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Clinical Context Is Essential

FENa and FEUrea are screening tools, not diagnostic tests. They should always be interpreted alongside urine microscopy (muddy brown casts = ATN; RBC casts = glomerulonephritis), urine output trends, serum BUN-to-creatinine ratio, and the clinical history. For comprehensive renal assessment, also consider the GFR calculator to stage the severity of kidney injury according to KDIGO criteria.

Prerenal AKI: Reversibility and Treatment

Prerenal AKI (FENa <1%) is the most common form of AKI and is often fully reversible with prompt treatment of the underlying cause. For volume depletion, IV crystalloid resuscitation (0.9% NS or LR) is first-line. For cardiogenic or hepatic causes, treatment targets the underlying condition. Prerenal AKI typically reverses within 24–72 hours of volume restoration if caught before structural tubular damage occurs. Prolonged or severe prerenal injury can progress to ischemic ATN, which takes 1–3 weeks to recover and may not fully reverse.

Urine Microscopy Complements FENa

Urine microscopy is the complement to FENa in AKI workup. Granular casts (muddy brown) indicate ATN — the result of sloughed tubular epithelial cells. Hyaline casts are nonspecific and seen in prerenal states. Red blood cell (RBC) casts are pathognomonic for glomerulonephritis. White blood cell (WBC) casts suggest pyelonephritis or acute interstitial nephritis (AIN). A normal microscopy in the setting of AKI and FENa <1% strongly supports prerenal etiology. For patients with metabolic disorders alongside AKI, our corrected calcium calculator helps address common electrolyte abnormalities in renal failure.

Sources & References

Fractional Excretion of Sodium in Acute Kidney Injury — National Kidney Foundation
KDIGO Clinical Practice Guideline for Acute Kidney Injury — Kidney Disease: Improving Global Outcomes (KDIGO)
UpToDate: Diagnostic approach to the patient with acute kidney injury — American Society of Nephrology

FENa Calculator