Fractional Excretion of Sodium (FENa)
FENa (%) = (U_Na × P_Cr) / (P_Na × U_Cr) × 100 with creatinine in the same units.
Calculator
How it works
What this tool does
It applies the standard clearance ratio definition of FENa to help teach how urinary sodium handling relates to filtration and tubular function.
Formula
FENa (%) = (U_Na × P_Cr) / (P_Na × U_Cr) × 100
References
Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;235(23):2518-2519.
Dosing & care
Teaching patterns (not rules)
- Low FENa: classically taught with sodium-avid, hypoperfused kidneys when not on diuretics.
- Higher FENa: classically taught with tubular injury or diuretic effect—always interpret in clinical context.
Limitations
Loop/thiazide diuretics, CKD, contrast timing, non-oliguric states, and lab error can invalidate “classic” cutoffs. FEUrea or other indices may be used concurrently.
AKI context (teaching)
- FENa is one datum among urine chemistries, microscopy, volume status, hemodynamics, and medications.
- When diuretics confound sodium excretion, clinicians may also review FEUrea or sequential trends rather than a single value.
- Fluid plans and nephrology decisions should not hinge on this page alone.
Educational tool only—not for diagnosis or treatment decisions.