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Renal

Fractional Excretion of Sodium (FENa)

Fraction of filtered sodium excreted in urine — helps distinguish prerenal from intrinsic (ATN) acute kidney injury.

Scope: Validated in oliguric AKI. Diuretics, glycosuria, and pre-existing CKD invalidate the prerenal cutoff.

Inputs

FieldUnitNotes
Urine sodium mEq/L 0–300
Plasma sodium mEq/L 100–200
Urine creatinine mg/dL 1–1000
Plasma creatinine mg/dL 0.1–30

FAQ

What is the Fractional Excretion of Sodium (FENa)?

Fraction of filtered sodium excreted in urine — helps distinguish prerenal from intrinsic (ATN) acute kidney injury.

What inputs does the Fractional Excretion of Sodium (FENa) need?

Urine sodium (mEq/L), Plasma sodium (mEq/L), Urine creatinine (mg/dL), Plasma creatinine (mg/dL).

Are there any limitations?

Validated in oliguric AKI. Diuretics, glycosuria, and pre-existing CKD invalidate the prerenal cutoff.

Related guides

FENa vs FEUrea: Prerenal vs ATN When the Patient Is on Diuretics

Fractional excretion of sodium vs urea in pediatric AKI — the <1% prerenal cutoff, why FEUrea (<35%) is needed on diuretics, and the neonatal caveat.

Related Renal calculators

Estimated GFR (Bedside Schwartz)

Paediatric estimated GFR using the bedside Schwartz (2009) equation.

Urine Anion Gap

Urine anion gap to assess renal NH4+ excretion in normal-anion-gap metabolic acidosis.

Fractional Excretion of Urea (FEUrea)

Fraction of filtered urea excreted in urine — distinguishes prerenal from intrinsic AKI and stays valid in patients on diuretics.

Transtubular Potassium Gradient (TTKG)

Estimates the driving force for potassium secretion in the cortical collecting duct — used to work up hyper- and hypokalaemia.

Calculated Serum Osmolality

Estimates serum osmolality from sodium, glucose and urea — compare with a measured value to derive the osmolar gap.

Fractional Excretion of Magnesium (FEMg)

Fraction of filtered magnesium excreted in urine — distinguishes renal magnesium wasting from extrarenal loss in hypomagnesaemia.

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