Frequently asked questions
What is FENa (fractional excretion of sodium)?
FENa is the percentage of filtered sodium that the kidneys excrete into the urine. Because healthy tubules reabsorb most filtered sodium, the fraction that escapes reflects how well tubular reabsorption is working. It is computed as FENa (%) = (urine sodium × plasma creatinine) / (plasma sodium × urine creatinine) × 100. Clinicians use it mainly to help distinguish prerenal acute kidney injury (low perfusion, intact tubules) from intrinsic renal injury such as acute tubular necrosis (damaged tubules).
How is FENa calculated? Worked example
Take simultaneous blood and urine samples. Example: urine sodium 10 mEq/L, plasma sodium 140 mEq/L, urine creatinine 100 mg/dL, plasma creatinine 2.0 mg/dL. FENa = (10 × 2.0) / (140 × 100) × 100 = 20 / 14,000 × 100 = 0.14%. A second example: urine sodium 60 mEq/L, plasma sodium 135 mEq/L, urine creatinine 40 mg/dL, plasma creatinine 3.0 mg/dL gives (60 × 3.0) / (135 × 40) × 100 = 180 / 5,400 × 100 = 3.33%. This calculator performs the same computation and interprets the result.
How do I interpret my FENa result?
The classical teaching cutoffs are: below 1% suggests a prerenal pattern (intact tubules conserving sodium because perfusion is low, e.g. dehydration or low cardiac output); 1–2% is an indeterminate gray zone seen in either condition; above 2% suggests intrinsic renal injury, typically acute tubular necrosis (ATN), where damaged tubules waste sodium. Values exactly at 1% or 2% fall in the indeterminate zone. These are interpretive guides, not a diagnosis — always read them with the full clinical picture.
Why is FENa unreliable in patients on diuretics?
Loop and thiazide diuretics block tubular sodium reabsorption directly, so urine sodium rises independently of renal perfusion. This falsely elevates FENa and can make a prerenal patient look like they have ATN. Because of this, FENa should not be used while a diuretic is actively acting; the standard alternative is FEUrea, since urea handling is passive and far less affected by diuretics. If you tick the diuretic checkbox above, the calculator warns you and points to the FEUrea tab.
What is FEUrea and how is it interpreted?
FEUrea (fractional excretion of urea) applies the same ratio idea to urea: FEUrea (%) = (urine urea × plasma creatinine) / (plasma urea × urine creatinine) × 100. The accepted cutoffs are: 35% or below suggests a prerenal pattern; above 50% suggests intrinsic renal injury; 35–50% is indeterminate. It is the preferred index when diuretics make FENa uninterpretable, and it uses a spot urine sample just like FENa — no 24-hour collection is needed.
Which units should I enter — mEq/L, mmol/L, mg/dL, µmol/L?
Sodium: mEq/L and mmol/L are identical for sodium because it is monovalent, so either label is fine. Creatinine: enter urine and plasma creatinine in the same unit — either mg/dL or µmol/L (µmol/L ÷ 88.4 = mg/dL). Urea (FEUrea tab): enter urine and serum urea in the same unit, either mg/dL (as BUN) or mmol/L. Because each formula is a ratio of ratios, consistent units cancel out; the calculator requires the urine and blood units to match to catch entry errors.
Can FENa alone diagnose the cause of acute kidney injury?
No. FENa is an adjunct, not a standalone diagnosis. It can be misleading in chronic kidney disease (baseline sodium wasting raises FENa), early glomerulonephritis, sepsis, rhabdomyolysis, and contrast-induced injury — all of which can show a low FENa despite intrinsic disease. Urine microscopy (e.g. muddy-brown granular casts in ATN), urine osmolality, the rate of creatinine change, volume status, and medication review all belong in the assessment. Treat the result as one data point for a clinician to weigh.
Do I need a 24-hour urine collection for FENa or FEUrea?
No. Both indices are designed for a random spot urine sample paired with a blood sample drawn at about the same time. The formulas compare concentration ratios, so total urine volume is irrelevant. Prompt processing matters more than collection duration — ideally within a couple of hours so the analytes do not degrade.
What does a FENa between 1% and 2% mean?
The 1–2% band is the indeterminate or gray zone: it is seen in both prerenal azotemia and early intrinsic renal injury, so neither can be confidently diagnosed from the number alone. In this range, supplement the result with FEUrea, urine sodium concentration, urine microscopy, clinical volume assessment, and how fast the serum creatinine is rising. Nephrology input is often valuable when the indices sit in the gray zone.
Is this calculator medical advice?
No. This tool is an educational estimate only — it computes the standard formulas and applies the classical teaching cutoffs. It is not medical advice, it does not diagnose acute kidney injury, and it cannot replace clinical judgment. Interpret every result with the full clinical context and consult a nephrologist or physician for decisions about real patients.