Calculate the Fractional Excretion of Sodium (FENa) to differentiate between prerenal and intrinsic renal failure. Enter urine sodium (U Na), urine creatinine (U Cr), plasma sodium (P Na), and plasma creatinine (P Cr) — and get back the FENa percentage along with a clinical interpretation: Pre-Renal, Intrinsic Renal, or Post-Renal.
FENa
--
Clinical Interpretation
--
Category
--
FENa stands for Fractional Excretion of Sodium. It measures the percentage of sodium filtered by the kidneys that is ultimately excreted in the urine. This ratio helps clinicians determine whether acute kidney injury (AKI) is caused by a pre-renal, intrinsic renal, or post-renal process.
FENa is calculated as: FENa (%) = (U Na × P Cr) / (P Na × U Cr) × 100. Where U Na is urine sodium, P Cr is plasma creatinine, P Na is plasma sodium, and U Cr is urine creatinine.
A FENa below 1% typically indicates a pre-renal cause such as dehydration, congestive heart failure, or hepatorenal syndrome, where the kidneys are conserving sodium. A FENa between 1% and 3% suggests intrinsic renal failure (e.g., acute tubular necrosis). A FENa of 4% or greater may point toward post-renal obstruction or severe intrinsic renal damage.
Yes. FENa can be falsely low in patients who have received diuretics, as diuretics artificially increase urinary sodium excretion. It should also be interpreted with caution in patients with contrast nephropathy, myoglobinuria, acute glomerulonephritis, and early urinary tract obstruction, where FENa may be less than 1% despite intrinsic injury.
A low FENa (< 1%) is commonly seen in prerenal azotemia (e.g., hypovolemia, heart failure, cirrhosis), acute glomerulonephritis, hepatorenal syndrome, and acute partial urinary tract obstruction. In these states, the kidneys are actively reabsorbing sodium to restore perfusion.
FENa measures sodium excretion and is the standard marker for differentiating prerenal from intrinsic AKI. FEUrea (fractional excretion of urea) is a useful alternative in patients on diuretics, since urea excretion is less affected by diuretic use. A FEUrea below 35% suggests prerenal AKI.
Urine sodium (U Na) and plasma sodium (P Na) are measured in mEq/L (milliequivalents per liter). Both urine creatinine (U Cr) and plasma creatinine (P Cr) are typically measured in mg/dL (milligrams per deciliter). Using consistent units ensures the creatinine values cancel out in the formula, yielding a valid percentage.
No. FENa is a helpful screening tool but should be used alongside clinical context, urine microscopy, patient history, and other lab values. A nephrologist's interpretation, along with assessment of volume status and medication history, is essential for accurate diagnosis of acute kidney injury.