Question 1

What is the FeNa formula?

Accepted Answer

FeNa (%) = (urine sodium × serum creatinine) / (serum sodium × urine creatinine) × 100. The intuition: it is the percentage of filtered sodium that ends up in the urine. A healthy, dehydrated kidney reabsorbs almost all filtered sodium, producing FeNa < 1%; an injured tubule cannot, producing FeNa > 2%.

Question 2

When is FeUrea more useful than FeNa?

Accepted Answer

When the patient has received a diuretic in the last 24–48 hours. Loop and thiazide diuretics force urinary sodium loss, falsely raising FeNa toward the "intrinsic" range even when the kidney itself is intrinsically intact. Urea reabsorption is largely passive and not affected by these diuretics, so FeUrea remains interpretable.

Question 3

What FeNa value indicates pre-renal AKI?

Accepted Answer

FeNa < 1% is the classic pre-renal cutoff. Values between 1% and 2% are indeterminate. FeNa > 2% suggests intrinsic AKI, most commonly acute tubular necrosis. The cutoffs are not absolute — clinical context, urine sediment, and the trajectory of creatinine all matter.

Question 4

What FeUrea value indicates pre-renal AKI?

Accepted Answer

FeUrea < 35% suggests pre-renal physiology even on diuretics. 35–50% is indeterminate. > 50% suggests intrinsic AKI. The Carvounis 2002 study found FeUrea <35% had ~90% sensitivity and ~95% specificity for pre-renal AKI in patients on diuretics.

Question 5

Can FeNa be low in intrinsic AKI?

Accepted Answer

Yes. Several intrinsic AKIs classically produce low FeNa: contrast nephropathy, rhabdomyolysis, hepatorenal syndrome, acute glomerulonephritis, and sometimes the early phase of any AKI before the tubule has fully failed. A low FeNa in the wrong clinical context is a clue, not a verdict.

Question 6

Do I need a 24-hour urine for FeNa?

Accepted Answer

No. FeNa and FeUrea are dimensionless ratios that cancel the urine flow rate, so a spot urine is sufficient. The calculation requires only urine sodium, urine creatinine, serum sodium, and serum creatinine — all standard chemistries.

Fractional Excretion of Sodium / Urea

References

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