FENa Calculator - Fractional Excretion of Sodium
Calculate the fractional excretion of sodium to differentiate between prerenal, intrinsic renal, and postrenal causes of acute kidney injury.
What is FENa and what does it measure?
Fractional Excretion of Sodium (FENa) measures percentage of filtered sodium excreted in urine. Helps differentiate acute kidney injury (AKI) causes: prerenal (decreased kidney perfusion), intrinsic renal (kidney damage), or postrenal (obstruction). Formula: FENa = [(Urine Na * Plasma Cr) / (Plasma Na * Urine Cr)] * 100. Requires simultaneous blood and urine samples. FENa <1% suggests prerenal; >2% suggests intrinsic renal disease.
How do you interpret FENa results?
FENa <1%: Prerenal AKI (dehydration, heart failure, hypotension). Kidneys avidly retain sodium. FENa 1-2%: Gray zone, could be either prerenal or intrinsic. Clinical context crucial. FENa >2%: Intrinsic renal (ATN, interstitial nephritis) or postrenal. Kidneys unable to reabsorb sodium. Exception: FENa <1% can occur in contrast nephropathy, rhabdomyolysis, or glomerulonephritis despite intrinsic damage. Always interpret with clinical picture and urinalysis.
When should FENa not be used?
Don't use FENa if: Patient on diuretics (artificially elevates FENa, use FEUrea instead), chronic kidney disease (unreliable), recent contrast exposure, baseline high urine output states. Diuretics increase sodium excretion regardless of cause, making FENa >1% even in prerenal states. Wait 12-24 hours after last diuretic dose, or calculate Fractional Excretion of Urea (FEUrea <35% = prerenal) which isn't affected by diuretics.
What is the difference between FENa and FEUrea?
FENa: Affected by diuretics, threshold <1% for prerenal. FEUrea: Not affected by diuretics (urea reabsorption passive, not affected by loop/thiazide), threshold <35% for prerenal, preferred in patients on diuretics. Formula similar: FEUrea = [(Urine Urea * Plasma Cr) / (Plasma Urea * Urine Cr)] * 100. Use FEUrea when: Recent diuretic use, unclear diuretic history, need to distinguish prerenal vs ATN in ICU patient on diuretics.
What are other causes of low FENa besides prerenal AKI?
Low FENa (<1%) can occur in: Prerenal AKI (most common - dehydration, CHF, cirrhosis with hepatorenal syndrome), contrast-induced nephropathy (intrinsic but FENa low), early rhabdomyolysis/myoglobinuria, acute glomerulonephritis, early obstruction, sepsis-induced AKI. High FENa (>2%): ATN, acute interstitial nephritis, post-obstructive diuresis, intrinsic kidney disease. FENa is guide, not absolute - combine with BUN/Cr ratio, urinalysis, clinical context for accurate diagnosis.