FENa Calculator
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What is the FENa Calculator?
The **Fractional Excretion of Sodium (FENa)** is a widely used calculation in nephrology to accurately help determine the cause of Acute Kidney Injury (AKI). Specifically, it helps differentiate between **prerenal azotemia** (decreased blood flow to the kidneys) and **Acute Tubular Necrosis (ATN)** (intrinsic kidney damage).
This free online FENa calculator computes the percentage of sodium filtered by the kidney that is excreted in the urine, giving you an immediate assessment of renal function.
Understanding the Formula
The FENa calculation uses serum and urine measures of sodium and creatinine. The standard formula is:
- Urine Na (UNa): Sodium concentration in urine (mmol/L)
- Plasma Cr (PCr): Serum creatinine level (mg/dL)
- Plasma Na (PNa): Serum sodium level (mmol/L)
- Urine Cr (UCr): Urine creatinine level (mg/dL)
Interpretation of FENa Results
The calculated percentage helps categorize the type of renal failure:
| FENa Value | Likely Etiology | Description |
|---|---|---|
| < 1% | Prerenal Disease | Kidneys are reabsorbing sodium to preserve volume (e.g., dehydration, heart failure). |
| > 1% (usually > 2%) | Intrinsic Renal (ATN) | Tubular damage prevents sodium reabsorption. |
| > 4% | Postrenal Disease | Obstruction in the urinary tract. |
Note: FENa is most useful in oliguric patients (urine output < 400ml/day). It may be less accurate if the patient is on diuretics, has chronic kidney disease, or has non-oliguric AKI.