# FENa Calculator (Fractional Excretion of Sodium)

Calculate Fractional Excretion of Sodium (FENa) from serum and urine lab values. Differentiate prerenal azotemia from intrinsic renal causes of acute kidney injury.

## What this calculates

The Fractional Excretion of Sodium (FENa) is one of the most useful calculations in nephrology for sorting out why a patient's kidneys are failing. It compares how much sodium the kidneys are filtering versus how much they are excreting, which tells you whether the tubules are working properly.

## Inputs

- **Serum Sodium** (mEq/L) — min 100, max 180 — Normal range: 136-145 mEq/L
- **Serum Creatinine** (mg/dL) — min 0.1, max 30 — Normal range: 0.6-1.2 mg/dL
- **Urine Sodium** (mEq/L) — min 1, max 300 — From a spot urine sample
- **Urine Creatinine** (mg/dL) — min 1, max 500 — From a spot urine sample

## Outputs

- **FENa** — Fractional Excretion of Sodium
- **Interpretation** — formatted as text — Most likely cause of acute kidney injury
- **Clinical Context** — formatted as text — Important caveats and limitations

## Details

The FENa formula is: **FENa = (Urine Na x Serum Cr) / (Serum Na x Urine Cr) x 100**

This calculation requires a simultaneous serum and spot urine sample. You do not need a 24-hour urine collection.

**How to interpret FENa:**

- **FENa  2%:** The tubules have lost their ability to reabsorb sodium, suggesting intrinsic kidney damage. Acute tubular necrosis (ATN) is by far the most common cause, usually from ischemia or nephrotoxins.

**Important limitations:**

- Diuretics invalidate FENa by forcing sodium excretion regardless of renal perfusion. In patients on diuretics, use Fractional Excretion of Urea (FEUrea) instead.
- FENa can be misleadingly low (below 1%) in contrast nephropathy, rhabdomyolysis, early obstruction, and acute glomerulonephritis, even though these are intrinsic renal processes.
- In chronic kidney disease, baseline FENa may already be elevated, reducing its diagnostic utility.

**Disclaimer:** This calculator is for educational purposes only. Always interpret lab values in the full clinical context with your healthcare team.

## Frequently Asked Questions

**Q: When should I use FENa vs FEUrea?**

A: Use FENa when the patient is NOT on diuretics. Diuretics force the kidneys to excrete sodium, which raises the FENa regardless of the underlying cause, making it unreliable. In patients on diuretics, use Fractional Excretion of Urea (FEUrea) instead. Urea reabsorption is less affected by diuretics. A FEUrea below 35% suggests prerenal azotemia, while above 50% suggests intrinsic renal disease.

**Q: Can FENa be used in chronic kidney disease?**

A: FENa is most useful in acute kidney injury (AKI), not chronic kidney disease (CKD). In CKD, the remaining nephrons compensate by excreting more sodium per nephron, so FENa is often elevated at baseline. This makes it difficult to distinguish a prerenal insult from the underlying chronic disease. Clinical context, trends in creatinine, and urine output are more helpful in CKD patients with acute-on-chronic injury.

**Q: What does an indeterminate FENa (1-2%) mean?**

A: An FENa between 1% and 2% falls in a gray zone that does not reliably distinguish prerenal from intrinsic causes. This can happen during the transition from prerenal to intrinsic injury, in mixed pathology, or in conditions like contrast nephropathy and early ATN. Additional data points like urine microscopy, BUN-to-creatinine ratio, urine osmolality, and clinical trajectory help clarify the picture.

---

Source: https://vastcalc.com/calculators/health/fena
Category: Health & Fitness
Last updated: 2026-04-08