Fractional Excretion of Sodium Calculator
Estimate the fractional excretion of sodium (FENa) from urine and serum sodium and creatinine values, and see whether your result points to a prerenal or an intrinsic cause of acute kidney injury.
Evaluate Prerenal vs Intrinsic Kidney Injury from Urine and Blood Values
FENa Formula
Where:
- \(U_{\text{Na}}\) — urine sodium concentration (mmol/L, same numeric value in mEq/L)
- \(S_{\text{Na}}\) — serum sodium concentration (mmol/L)
- \(U_{\text{Cr}}\) — urine creatinine concentration (mg/dL or µmol/L)
- \(S_{\text{Cr}}\) — serum creatinine concentration (mg/dL or µmol/L)
The formula compares the amount of sodium the kidneys clear relative to the amount of creatinine they clear, expressed as a percentage of the filtered sodium load. Because the urine and serum terms of each solute appear as ratios, any consistent concentration units cancel out.
FENa – Calculation Example
Example: a patient with acute kidney injury has a urine sodium of 40 mmol/L, a serum sodium of 138 mmol/L, a urine creatinine of 60 mg/dL and a serum creatinine of 2 mg/dL.
\[ \text{FENa} = \frac{40 \times 2}{138 \times 60} \times 100 = \frac{80}{8280} \times 100 \approx 0.97\% \]
Result: FENa ≈ 0.97%, which is below 1% and suggests a prerenal (hemodynamic) cause of acute kidney injury, such as volume depletion or heart failure.
How to Use the FENa Calculator
- Enter the Urine sodium value from a spot (random) urine sample in mmol/L (the number is identical in mEq/L).
- Enter the Serum sodium value from the blood drawn at roughly the same time, in mmol/L.
- Enter the Urine creatinine value and select its unit — mg/dL or µmol/L — using the Unit dropdown next to the field.
- Enter the Serum creatinine value from the same time point and select its unit the same way.
- Read your Fractional excretion of sodium in percent, together with an interpretation (prerenal, indeterminate, or intrinsic renal injury) and a short explanation of what the result means.
If you enter creatinine in µmol/L, the calculator automatically converts it to mg/dL (1 mg/dL = 88.4 µmol/L), so both units give the same FENa.
What Is the Fractional Excretion of Sodium?
FENa is the percentage of the sodium filtered by the kidneys that ends up being excreted in the urine. In a healthy person on a normal diet, the kidneys reabsorb more than 99% of filtered sodium, so FENa is typically well below 1%. The classic clinical use of FENa is to help distinguish between the two major categories of acute kidney injury (AKI): prerenal and intrinsic renal.
In a prerenal state — volume depletion, hemorrhage, heart failure, cirrhosis — the kidney itself is intact. It senses reduced perfusion and mounts a maximal sodium-conserving response, driving FENa below 1%. In intrinsic injury, most commonly acute tubular necrosis (ATN) from ischemia, sepsis or nephrotoxins, the tubular cells are damaged and can no longer reabsorb sodium effectively, so FENa rises above 2%. Values between 1% and 2% are considered indeterminate and must be interpreted with the wider clinical picture.
A low FENa suggests that renal function may improve rapidly once the underlying hemodynamic problem is corrected, for example with intravenous fluids. A high FENa points toward intrinsic damage, where management is supportive — removing the offending drug, treating sepsis, and maintaining electrolyte and fluid balance — rather than aggressive volume expansion. FENa is a bedside clue, not a diagnosis; it should always be weighed together with the history, urine output, urinalysis, and the response to treatment.
Assumptions, Limitations and Practical Tips
- Diuretics invalidate FENa. Loop and thiazide diuretics force sodium into the urine and can push FENa above 2% even when the true state is prerenal. If the patient has received diuretics recently, prefer the fractional excretion of urea (FEUrea), where a value below 35% suggests a prerenal cause.
- Chronic kidney disease distorts the result. In CKD the baseline FENa is already elevated, so a "high" value does not necessarily indicate acute tubular necrosis. Check the result against the patient's baseline kidney function — our eGFR calculator (CKD-EPI 2021) can help estimate it.
- Some specific conditions give misleadingly low values. Contrast nephropathy, rhabdomyolysis and glomerulonephritis can produce a low FENa despite intrinsic injury.
- Timing and stability matter. FENa is most reliable in oliguric AKI with stable (not rapidly changing) kidney function, using a urine and blood sample collected at the same time point.
- Hyponatremia or hypernatremia? Interpret the serum sodium itself alongside FENa; a corrected value may be needed in hyperglycemia — see the corrected sodium calculator. For a broader metabolic picture, the serum osmolality calculator is a useful companion, and in liver disease the MELD-Na score incorporates sodium into prognosis.
The thresholds of less than 1% and greater than 2% come from the classic studies by Espinel (JAMA, 1976) and Miller et al. (Annals of Internal Medicine, 1978). This calculator is provided for information and education only and is not a substitute for clinical judgment or a medical diagnosis.
Frequently Asked Questions
How do I calculate FENa step by step without a calculator?
Multiply the urine sodium by the serum creatinine, multiply the serum sodium by the urine creatinine, then divide the first product by the second and move the decimal point two places to convert to percent. For example, with urine sodium 22 mmol/L, serum creatinine 1.8 mg/dL, serum sodium 140 mmol/L and urine creatinine 90 mg/dL: \( (22 \times 1.8) \div (140 \times 90) = 39.6 \div 12600 \approx 0.0031\), so FENa ≈ 0.31%.
What is a normal FENa value in a healthy person?
In healthy people eating a normal amount of salt, FENa usually sits around 0.5%–1%, because the kidneys reabsorb more than 99% of the filtered sodium. On a very low-sodium diet or with certain hormone changes it can fall even lower. Note that these reference values apply to stable kidney function — the 2% cutoffs used here are specifically for classifying acute kidney injury, not for judging everyday kidney health.
Can I enter sodium in mEq/L or creatinine in µmol/L?
Yes. Sodium values in mEq/L are numerically identical to mmol/L, so you can type them in directly. For creatinine, just pick µmol/L in the unit dropdown next to the field; the tool converts it for you using 1 mg/dL = 88.4 µmol/L. What matters is that urine and serum creatinine use the same unit — the calculator handles that automatically, and any consistent units cancel in the ratio.
Do I need a 24-hour urine collection to measure FENa?
No, a single random (spot) urine sample is enough, provided blood is drawn at roughly the same time. FENa is a ratio of urine-to-serum concentrations for two solutes, so urine flow rate cancels out and no timed collection is required. That is exactly why it is popular at the bedside. A mismatch in timing between the urine and blood samples is, however, a common source of error, especially when kidney function is changing quickly.
What is the difference between FENa and FEUrea, and when should I use FEUrea instead?
FEUrea measures the same idea — how much of the filtered urea escapes into the urine — but urea reabsorption is far less affected by diuretics than sodium reabsorption is. If the patient has taken loop or thiazide diuretics in the previous hours, FENa becomes unreliable and a FEUrea below 35% points toward a prerenal cause instead. FENa remains the standard choice when no diuretics have been given and the patient is oliguric with stable kidney function.
What are the most common mistakes that give a wrong FENa result?
The biggest ones are: entering creatinine in µmol/L while the unit is still set to mg/dL (which inflates the result roughly 88-fold); using urine and blood samples drawn hours apart during rapidly changing kidney function; ignoring recent diuretic use; and applying the thresholds to patients with chronic kidney disease, where baseline FENa is already higher. Double-check the unit dropdowns and always read the number alongside the clinical picture, not in isolation.
Does a low FENa always mean the kidney injury is reversible?
Usually it suggests a hemodynamic cause that can improve with fluids or treatment of the underlying problem, but not always. Contrast-induced nephropathy, rhabdomyolysis and some forms of glomerulonephritis can produce a low FENa despite genuine intrinsic damage. Also, if a prerenal state is left untreated long enough, it can progress to acute tubular necrosis. FENa is one clue among several — urine output, urinalysis, urine microscopy and the response to treatment all need to be considered.
nice job on this post