What FENa value indicates pre-renal AKI?

A FENa below 1% generally indicates pre-renal azotemia, where the kidneys are retaining sodium due to reduced renal perfusion. Tubular function is intact and the kidneys are responding normally to low flow.

What FENa value suggests acute tubular necrosis?

A FENa above 2% is consistent with intrinsic renal injury such as acute tubular necrosis (ATN), where tubular damage impairs sodium reabsorption and more sodium is excreted.

Is FENa reliable in patients taking diuretics?

No. Loop and thiazide diuretics increase urinary sodium excretion, which falsely elevates FENa even in pre-renal states. In diuretic-treated patients, the Fractional Excretion of Urea (FEUrea) is a more reliable marker.

What does a FENa between 1% and 2% mean?

An intermediate value (1–2%) is indeterminate and must be interpreted in clinical context. It can occur in early ATN, contrast nephropathy, myoglobinuria, or mixed pre-renal and intrinsic disease.

What units should I use for the inputs?

Urine and plasma sodium should be in mEq/L (or mmol/L, which is numerically equivalent). Urine and plasma creatinine should both be in the same units — typically mg/dL. Using consistent units for both creatinine values is essential since they cancel out in the formula.

Can FENa be used in chronic kidney disease?

FENa is less reliable in CKD because baseline tubular dysfunction may alter sodium handling independent of the acute insult. Clinical judgment and trends over time are important in CKD patients.

When can you not use FENa?

FENa is unreliable in patients receiving diuretics (loop or thiazide agents), non-oliguric states, pre-existing chronic kidney disease, acute glomerulonephritis, or contrast-induced AKI. In patients taking diuretics, the Fractional Excretion of Urea (FEUrea) is commonly assessed instead.

FENa Calculator

Enter urine and plasma sodium and creatinine values to calculate FENa and interpret acute kidney injury etiology.

Lab Values

Urine

Urine Sodium (U_Na)

mEq/L

Urine Creatinine (U_Cr)

mg/dL

Plasma (Serum)

Plasma Sodium (P_Na)

mEq/L

Plasma Creatinine (P_Cr)

mg/dL

Interpretation Reference

FENa < 1%

Pre-renal azotemia — intact tubular function, reduced renal perfusion. Likely responds to volume resuscitation.

FENa 1–2%

Indeterminate — consider clinical context, diuretic use, CKD, contrast nephropathy, or early ATN.

FENa > 2%

Intrinsic renal injury — acute tubular necrosis (ATN) or other intrinsic renal disease. Tubular sodium reabsorption impaired.

Note: FENa is unreliable in patients receiving diuretics. Consider FEUrea (<35% pre-renal) in those cases.

Summary

The Fractional Excretion of Sodium (FENa) measures the percentage of filtered sodium that is excreted in the urine. It is a key diagnostic tool in acute kidney injury (AKI), helping clinicians determine whether the cause is pre-renal (decreased perfusion), intrinsic renal (tubular damage), or post-renal (obstruction). FENa below 1% typically indicates pre-renal AKI, while FENa above 2% suggests intrinsic renal disease such as acute tubular necrosis.

How it works

Collect a spot urine sample and a simultaneous blood sample.
Measure urine sodium (mEq/L), urine creatinine (mg/dL), plasma sodium (mEq/L), and plasma creatinine (mg/dL).
Enter all four values into the calculator fields.
The tool applies the formula: FENa (%) = (urine Na × plasma Cr) / (plasma Na × urine Cr) × 100.
Review the calculated percentage alongside its clinical interpretation.
Note that FENa may be unreliable in patients on diuretics — consider FEUrea in those cases.

Use cases

Differentiate pre-renal AKI from acute tubular necrosis (ATN) at the bedside.
Guide fluid resuscitation decisions in oliguria.
Evaluate AKI in the emergency department or ICU setting.
Assess renal tubular function in suspected nephrotoxin exposure.
Screen for hepatorenal syndrome in cirrhosis patients.
Interpret azotemia in post-operative patients.
Support nephrology consult discussions with objective data.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu