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FENa Calculator

Calculate Fractional Excretion of Sodium for acute kidney injury diagnosis

Serum Sodium (mEq/L)

Urine Sodium (mEq/L)

Serum Creatinine (mg/dL)

Urine Creatinine (mg/dL)

⚠️ Medical Disclaimer: This calculator is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional before making health-related decisions.

About FENa Calculator

The Fractional Excretion of Sodium (FENa) Calculator is a vital diagnostic tool used by healthcare professionals to evaluate acute kidney injury (AKI). This calculator helps differentiate between various causes of sudden kidney function decline by measuring the percentage of filtered sodium that is excreted in urine. Understanding FENa values is crucial for determining appropriate treatment strategies and improving patient outcomes.

What is FENa?

FENa represents the fraction of sodium filtered by the kidneys that appears in the urine. In healthy kidneys with adequate blood flow, most filtered sodium is reabsorbed back into the bloodstream, resulting in low FENa values. When kidney function is compromised, FENa patterns change depending on the underlying cause, making it an invaluable diagnostic marker.

How FENa Works

The FENa calculation requires four laboratory values: serum sodium, urine sodium, serum creatinine, and urine creatinine. These values are obtained from routine blood and urine tests. The formula compares the ratio of sodium clearance to creatinine clearance, providing insight into how the kidneys are handling sodium filtration and reabsorption.

Clinical Interpretation

FENa <1%: Indicates pre-renal acute kidney injury. The kidneys are functioning properly but receiving inadequate blood flow due to conditions like dehydration, heart failure, or shock.
FENa 1-4%: Suggests intrinsic renal disease such as acute tubular necrosis (ATN), acute interstitial nephritis, or glomerulonephritis affecting the kidney tissue directly.
FENa >4%: Points to post-renal obstruction where urine flow is blocked by conditions like kidney stones, enlarged prostate, or bilateral ureter obstruction.

Important Limitations

While FENa is highly useful, it has important limitations. It's only valid for acute kidney injury, not chronic kidney disease. Diuretic medications significantly affect sodium excretion and invalidate FENa results. In patients on diuretics, FEUrea (fractional excretion of urea) may be used as an alternative. Additionally, FENa should be interpreted alongside other clinical findings and laboratory tests for accurate diagnosis.

When to Use FENa

FENa is most valuable when a patient presents with sudden kidney function decline and the cause is unclear. It helps emergency physicians and nephrologists quickly differentiate between pre-renal azotemia (which may respond to fluid resuscitation), intrinsic kidney damage (requiring specific treatments), and urinary obstruction (potentially needing urgent intervention). Early accurate diagnosis through FENa can prevent irreversible kidney damage.

This calculator provides immediate FENa results for educational and clinical reference purposes. Always consult qualified healthcare professionals for medical diagnosis and treatment decisions. FENa is one component of comprehensive kidney function assessment and should be interpreted within the full clinical context.

Frequently Asked Questions

What is FENa and why is it important? +

FENa (Fractional Excretion of Sodium) is the percentage of filtered sodium that is excreted in urine. It's crucial for diagnosing acute kidney injury (AKI) by differentiating between pre-renal, intrinsic, and post-renal causes. FENa <1% suggests pre-renal causes, 1-4% indicates intrinsic renal disease, and >4% points to post-renal obstruction.

What lab values do I need to calculate FENa? +

To calculate FENa, you need four lab values: serum sodium (mEq/L), urine sodium (mEq/L), serum creatinine (mg/dL), and urine creatinine (mg/dL). These values are obtained from blood and urine tests ordered by your healthcare provider.

Can FENa be used for chronic kidney disease? +

No, FENa is only useful for diagnosing acute kidney injury (AKI), not chronic kidney disease (CKD). It helps identify the cause of sudden kidney function decline but is not reliable for long-term kidney disease assessment.

Does diuretic use affect FENa results? +

Yes, diuretics significantly affect FENa results by increasing sodium excretion, which can falsely elevate FENa values. FENa cannot be accurately interpreted in patients currently taking diuretics. In such cases, FEUrea (fractional excretion of urea) may be used instead.

What does a FENa less than 1% indicate? +

FENa <1% typically indicates pre-renal acute kidney injury, meaning decreased blood flow to the kidneys. Common causes include dehydration, heart failure, shock, or renal artery stenosis. The kidneys are functioning properly but receiving inadequate blood supply.

What does a FENa greater than 4% mean? +

FENa >4% usually indicates post-renal acute kidney injury caused by urinary tract obstruction. This can result from conditions like benign prostatic hyperplasia (BPH), kidney stones, bladder stones, or bilateral ureter obstruction blocking urine flow.

Is this FENa calculator medically accurate? +

Yes, this calculator uses the standard medical formula: FENa = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100. However, it's for educational purposes only. Always consult healthcare professionals for medical diagnosis and treatment decisions.