Free FENa Calculator

The FENa Calculator and Its Role in AKI Evaluation

This free FENa AKI calculator (fractional excretion of sodium calculator) is available online to assist clinicians in differentiating the causes of acute kidney injury (AKI) and oliguria. By analyzing blood and urine sodium and creatinine levels, the tool helps identify whether renal impairment stems from prerenal, intrinsic, or postrenal pathology. This article explains what FENa represents, which lab values are required, how the calculation is performed, and how to interpret the results.

What Is Fractional Excretion of Sodium (FENa)?

FENa indicates the proportion of filtered sodium that is excreted via the urine, given as a percentage. In the setting of acute renal failure, it is widely utilized to separate prerenal disease (caused by decreased renal perfusion) from acute tubular necrosis (ATN), an intrinsic form of kidney injury.

Limitations of FENa

The clinical utility of FENa has several restrictions. It is only reliable in acute renal failure and is not validated for chronic kidney disease. Additionally, concurrent use of diuretics can alter sodium handling and render the FENa value inaccurate, so results must be interpreted cautiously in patients on such medications.

Required Laboratory Results

To use the FENa calculator, four laboratory values are necessary:

• Serum creatinine (PCr): measured in mg/dL. Normal ranges are approximately 0.6–1.2 mg/dL for adult males and 0.5–1.1 mg/dL for adult females.
• Serum sodium (PNa): also called plasma sodium, expressed in mmol/L.
• Urine creatinine (UCr): can be obtained from a random sample or a 24‑hour urine collection; usually reported in mg/dL.
• Urine sodium (UNa): typically measured in a 24‑hour urine collection, with a reference range of 100–260 mmol/L per 24 hours.

Ensure that the units for both creatinine values (serum and urine) are the same, and both sodium values share the same unit, to obtain a valid FENa result.

The FENa Formula

The calculation follows a simple ratio:

$$\text{FENa} = \frac{\text{PCr} \times \text{UNa}}{\text{PNa} \times \text{UCr}} \times 100\%$$

Where:

• PCr = plasma creatinine (mg/dL)
• PNa = plasma sodium (mmol/L)
• UCr = urine creatinine (mg/dL)
• UNa = urine sodium (mmol/L)

Example Calculation

Suppose a patient presents with the following values:

• Serum creatinine: 1.5 mg/dL
• Serum sodium: 140 mmol/L
• Urine creatinine: 100 mg/dL
• Urine sodium: 20 mmol/L

$$\text{FENa} = \frac{1.5 \times 20}{140 \times 100} \times 100\% = \frac{30}{14000} \times 100\% \approx 0.21\%$$

A FENa below 1% suggests a prerenal process.

Interpreting FENa Results

FENa values guide the differential diagnosis of AKI:

FENa Value	Most Likely Etiology
< 1%	Prerenal cause
1% – 4%	Intrinsic renal cause
> 4%	Postrenal cause

• Prerenal AKI results from conditions that reduce renal blood flow, such as hypovolemia, congestive heart failure, renal artery stenosis, or sepsis.
• Intrinsic AKI involves direct damage to the kidney parenchyma, including ATN, acute interstitial nephritis (AIN), and glomerulonephritis.
• Postrenal AKI usually follows urinary tract obstruction, for example benign prostatic hyperplasia (BPH), bladder calculi, or bilateral ureteral obstruction.

Important Disclaimer

While this FENa medical calculator provides accurate estimates based on the input data, it is intended for educational and clinical reference purposes only. It does not replace a comprehensive professional assessment. Any health concerns should be discussed with a qualified physician.