Healthcare & Clinical
Fractional Excretion of Sodium (FENa) & Urea (FEUrea) in Acute Kidney Injury
Calculate FENa (%) = (U_Na * P_Cr) / (P_Na * U_Cr) * 100 and diuretic-resistant FEUrea (%) to differentiate prerenal azotemia from intrinsic acute tubular necrosis (ATN).
Distinguish volume-responsive prerenal AKI from tubular injury!
When serum creatinine rises acutely, intact renal tubules respond to hypoperfusion (prerenal azotemia) by avidly reabsorbing filtered sodium and water—driving Fractional Excretion of Sodium FENa = ((U_Na * P_Cr) / (P_Na * U_Cr)) * 100 below 1.0%! Conversely, damaged tubular epithelium in Acute Tubular Necrosis (ATN) leaks sodium into urine, pushing FENa above 2.0%!
What if the patient received loop or thiazide diuretics, which artificially elevate urinary sodium even during severe hypovolemia? Switch to Fractional Excretion of Urea FEUrea = ((U_Urea * P_Cr) / (BUN * U_Cr)) * 100: proximal urea reabsorption remains preserved under loop diuretics, so FEUrea <= 35% still reliably identifies prerenal hypoperfusion (whereas FEUrea > 50% points to intrinsic ATN)!
| AKI Clinical Presentation | U_Na / P_Na (mEq/L) | U_Cr / P_Cr (mg/dL) | FENa (%) & Diagnostic Etiology |
|---|---|---|---|
| Dehydration / Hypovolemia | 12 / 140 | 120 / 2.4 | 0.17% (< 1% -> Prerenal AKI) |
| Indeterminate Borderline Zone | 28 / 140 | 48 / 2.4 | 1.00% (1%–2% -> Indeterminate) |
| Ischemic Acute Tubular Necrosis | 55 / 140 | 45 / 2.4 | 2.10% (> 2% -> Intrinsic ATN) |
Fractional Excretion of Sodium FENa (%) Formula
Computes FENa (%) from spot urine sodium B2 (mEq/L), plasma sodium B3 (mEq/L), urine creatinine B4 (mg/dL), and plasma creatinine B5 (mg/dL); note that U_Cr and P_Cr must use matching concentration units.
=IF(OR(B3 <= 0, B4 <= 0), "Invalid Plasma Na or Urine Cr", ROUND(((B2 * B5) / (B3 * B4)) * 100, 2))Clinical AKI Etiology Band (< 1% Prerenal, 1–2% Indeterminate, > 2% ATN)
Classifies FENa into standard nephrology diagnostic thresholds.
=IF(OR(B3 <= 0, B4 <= 0), "Invalid Lab Input", IFS(((B2 * B5) / (B3 * B4)) * 100 < 1, "Prerenal Azotemia (< 1%)", ((B2 * B5) / (B3 * B4)) * 100 <= 2, "Indeterminate (1% - 2%)", TRUE, "Intrinsic ATN (> 2%)"))Try the example
Use the original values or change them to test your understanding.