FENa Calculator Fractional Excretion of Sodium

1FENa Explained — Distinguishing AKI Types

When FENa Can Be Misleading

False low FENa (looks prerenal, but isn't):

• Early contrast nephropathy — tubules initially intact, FENa <1% for first 24 hours
• Myoglobinuria / haemoglobinuria — tubular toxin but early FENa low
• Early urinary obstruction
• Acute glomerulonephritis — sodium retention due to inflammation

False high FENa (looks intrinsic, but may be prerenal):

• Diuretics — loop and thiazide diuretics block tubular sodium reabsorption → use FEUrea instead
• Pre-existing CKD — tubular function already impaired
• Adrenal insufficiency
• Bicarbonaturia (high bicarbonate with sodium in urine)

AKI Management by Type

• Prerenal (FENa <1%): IV fluid resuscitation — 500 mL NS over 30 min (adult). Monitor urine output, creatinine. Identify and treat cause (bleeding, sepsis, heart failure, medications)
• Intrinsic renal (FENa >2%): Identify specific cause — ATN (most common), glomerulonephritis, interstitial nephritis, vasculitis. Nephrology referral. Avoid nephrotoxins. Optimise haemodynamics
• Postrenal (obstruction): FENa may be >2% or <1% early. Urinary ultrasound mandatory. Catheterise if urethral obstruction. Nephrostomy if ureteric obstruction

2Frequently Asked Questions

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Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.