What it is
When a child is oliguric, the kidney’s handling of filtered solute tells you whether tubules are intact and avidly reabsorbing (prerenal hypoperfusion) or damaged and leaking (acute tubular necrosis, ATN). The fractional excretion of sodium (FENa) and of urea (FEUrea) quantify exactly that — each is the fraction of filtered solute that ends up in the urine.
The formulas
$$ FENa,(%) = \frac{U_{Na} \times P_{Cr}}{P_{Na} \times U_{Cr}} \times 100 $$
$$ FEUrea,(%) = \frac{U_{urea} \times P_{Cr}}{P_{urea} \times U_{Cr}} \times 100 $$
Both need a paired spot urine and serum: sodium (or urea) and creatinine in each. Plasma creatinine cancels the GFR term, isolating tubular handling.
When to use it
Use in oliguric AKI when the prerenal-versus-intrinsic distinction will change management (fluids vs renal-sparing care). The split is sharpest in oliguric states; in non-oliguric AKI the indices lose discrimination.
- FENa < 1% → prerenal (intact tubules reclaiming sodium). > 2% → ATN.
- FEUrea < 35% → prerenal. > 50% → ATN.
Worked example
Oliguric child: U_Na 10, serum Na 140, U_Cr 100, serum Cr 2.0 (consistent units).
$$ FENa = \frac{10 \times 2.0}{140 \times 100} \times 100 = 0.14% $$
FENa well under 1% — prerenal. The tubules are working; the problem is upstream perfusion.
Why FEUrea, and the diuretic trap
A loop or thiazide diuretic forces natriuresis, so a truly prerenal child on furosemide can show a FENa > 1% and be mislabelled as ATN. Urea handling is driven largely in the proximal tubule and is not blocked by loop diuretics, so FEUrea stays low (< 35%) in prerenal states even after diuretics — in the original series, 39 of 40 such patients had FEUrea < 35% despite FENa > 1%. Whenever a diuretic has been given, trust FEUrea over FENa.
Pitfalls and caveats
- Neonatal caveat. Newborns — especially preterm — have immature tubular sodium handling and a physiologically high baseline FENa. A FENa up to ~3% can be normal in term neonates and even higher in sick preterms, so the adult <1% / >2% bands do not apply. Interpret neonatal indices against gestational-age-specific norms.
- Confounders raise FENa independent of ATN: diuretics, CKD, glycosuria/osmotic diuresis, bicarbonaturia, high salt intake.
- Confounders lower FENa despite intrinsic injury: contrast nephropathy, pigment (rhabdomyolysis, haemoglobinuria) and early sepsis can give a “prerenal” FENa with established injury.
- Indices are an adjunct to history, volume assessment, and urine microscopy — never a standalone diagnosis.
Run it: Fractional Excretion of Sodium (FENa) · FEUrea
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.