FENa Calculator
Enter serum and urine sodium and creatinine values to calculate the fractional excretion of sodium (FENa). This calculator also computes FEUrea, a more reliable alternative for patients taking diuretics. Results include instant clinical interpretation with the standard prerenal, indeterminate, and intrinsic renal cut-offs, plus a step-by-step breakdown of the math.
Formula
Worked example
A patient presents with oliguria. Labs: serum Na 140 mEq/L, urine Na 20 mEq/L, serum Cr 2.0 mg/dL, urine Cr 120 mg/dL. FENa = (20 x 2.0) / (140 x 120) x 100 = 40 / 16800 x 100 = 0.24%. This is below 1%, consistent with prerenal azotemia - the tubules are avidly conserving sodium in response to decreased renal perfusion.
What is FENa and why does it matter?
The fractional excretion of sodium (FENa) measures what percentage of the sodium filtered by the glomeruli actually ends up in the urine, rather than being reabsorbed by the tubules. In healthy kidneys under conditions of normal volume status, roughly 99% of filtered sodium is reabsorbed. When blood flow to the kidneys drops - because of volume depletion, heart failure, or hepatorenal syndrome - the tubules kick into overdrive and reabsorb even more sodium, pushing FENa below 1%. When the tubular cells themselves are damaged, as in acute tubular necrosis (ATN), they lose the ability to hold on to sodium, and FENa climbs above 2%. This makes FENa one of the most useful single numbers for quickly narrowing down the cause of acute kidney injury (AKI) without expensive or invasive tests.
FENa formula and how this calculator works
FENa (%) = (Urine Na x Serum Creatinine) / (Serum Na x Urine Creatinine) x 100. Both creatinine values must be in the same units - this calculator accepts mg/dL or µmol/L and converts automatically. Sodium is always entered in mEq/L (equivalent to mmol/L for monovalent ions). FENa is calculated from a paired spot urine and serum sample - it does not require a timed 24-hour collection. For FEUrea, the same ratio is applied using urea nitrogen concentrations instead of sodium: FEUrea (%) = (Urine Urea x Serum Cr) / (Serum Urea x Urine Cr) x 100, with a cut-off of 35% instead of 1%.
When to use FEUrea instead of FENa
FENa loses its diagnostic value in patients who have received loop diuretics (furosemide, bumetanide) or thiazides, because diuretics force the kidney to excrete sodium even in a prerenal state, artificially pushing FENa above 1%. In these patients, FEUrea is more reliable because urea reabsorption is not directly affected by diuretics. A FEUrea below 35% still suggests prerenal physiology even after diuretic administration. Enter serum BUN and urine urea nitrogen in the optional fields above to get FEUrea alongside FENa.
Limitations and conditions that confound FENa
FENa is only valid in the setting of acute kidney injury (rising creatinine), not chronic kidney disease. Several conditions can give a falsely low FENa (below 1%) despite genuine tubular injury: early acute glomerulonephritis, contrast-induced nephropathy, myoglobinuric AKI (rhabdomyolysis), hemoglobinuric AKI, and acute urinary obstruction in its earliest phase. In these situations the tubules may still be functionally able to reabsorb sodium even as glomerular filtration falls. Conditions that can give a falsely elevated FENa (above 2%) when the underlying cause is actually prerenal include advanced chronic kidney disease with tubular dysfunction, salt-wasting nephropathy, and adrenal insufficiency. Always interpret FENa in the context of the clinical history, fluid balance, urine microscopy, and the trend in creatinine.
FENa and FEUrea clinical interpretation
| Marker | Value | Interpretation | Typical cause |
|---|---|---|---|
| FENa | < 1% | Prerenal | Volume depletion, heart failure, hepatorenal syndrome |
| FENa | 1-2% | Indeterminate | Mixed picture - needs clinical correlation |
| FENa | > 2% | Intrinsic renal | Acute tubular necrosis, interstitial nephritis |
| FEUrea | < 35% | Prerenal | Use when patient is on diuretics |
| FEUrea | >= 35% | Intrinsic renal | Tubular dysfunction despite diuretics |
Standard cut-offs for differentiating causes of acute kidney injury (AKI). FEUrea is preferred in patients taking diuretics.
Frequently asked questions
What is a normal FENa?
In healthy adults with normal kidney function and normal volume status, FENa is typically around 0.5-1%. In clinical practice, FENa is only measured when acute kidney injury is present. A value below 1% is taken as evidence of prerenal physiology; a value above 2% suggests intrinsic renal injury such as acute tubular necrosis. The range from 1% to 2% is diagnostically indeterminate and requires clinical correlation.
Can I use FENa in a patient who has taken furosemide?
No - diuretics invalidate FENa because they force urinary sodium excretion regardless of volume status, driving FENa above 1% even in a genuinely prerenal state. Use FEUrea instead (cut-off: 35%). Enter serum BUN and urine urea nitrogen in the optional fields of this calculator to get a simultaneous FEUrea result.
Why does contrast nephropathy give a low FENa despite tubular injury?
Iodinated contrast causes intense renal vasoconstriction in addition to direct tubulotoxicity. The severe reduction in renal blood flow activates sodium conservation mechanisms so powerfully that FENa remains below 1% even though the tubular cells are being damaged. The same mechanism applies in myoglobinuric AKI (rhabdomyolysis), where intense vasoconstriction overrides tubular dysfunction in the early hours.
What sample do I need to calculate FENa?
You need a paired spot (random) urine sample and a serum sample drawn at approximately the same time. No timed 24-hour collection is required. The four measurements needed are: serum sodium, urine sodium, serum creatinine, and urine creatinine. If you also want FEUrea, you need serum BUN and urine urea nitrogen from the same paired samples.
Is FENa useful in chronic kidney disease?
FENa is not validated for chronic kidney disease (CKD). In CKD, the damaged tubules have reduced ability to reabsorb sodium as part of the long-standing disease process, so FENa is often elevated at baseline and cannot reliably distinguish a prerenal superimposed on CKD from intrinsic worsening of CKD. FENa interpretation is only reliable in the context of a new or acute rise in creatinine.