How this instrument works
FENa compares how much of the filtered sodium load actually leaves the body in urine against how much gets reclaimed, using creatinine as the correction factor. Creatinine is filtered but barely reabsorbed or secreted by healthy tubules, so the ratio of urine to plasma creatinine tracks how much the fluid has been concentrated by water reabsorption; dividing sodium handling by that same correction cancels out urine concentration and leaves a measure of sodium handling specifically.
Espinel described the test in a 1976 JAMA paper, a small study of only seventeen patients, and it's worth stating exactly what that paper claimed rather than the version now taught in most classrooms. Espinel's original cutoffs were below 1% for a prerenal pattern — the kidney conserving sodium hard because blood flow to it has dropped — and above 3% for acute tubular necrosis, where damaged tubule cells can no longer reclaim what's been filtered. The 'above 2%' line repeated in many textbooks today is a later simplification of that finding, not what the 1976 paper itself reported.
The test loses its meaning in anyone taking a diuretic, since diuretics force sodium excretion regardless of the kidney's underlying state — a patient with genuinely reduced blood flow can still post a falsely high reading while on a loop diuretic. That single limitation is the reason fractional excretion of urea was later proposed as an alternative for exactly that situation.
- Enter Urine sodium (Una) in mEq/L and Plasma sodium (Pna) in mEq/L.
- Enter Plasma creatinine (Pcr) and Urine creatinine (Ucr), both in mg/dL.
- Read FENa as a percentage — under 1% leans prerenal, above 3% leans toward tubular injury, and the space between is a gray zone rather than a clean cutoff.
Worked example — Una 10, Pcr 2.0, Pna 140, Ucr 60
Multiply the top: 10 × 2.0 = 20. Multiply the bottom: 140 × 60 = 8400. Divide: 20 ÷ 8400 ≈ 0.00238, and ×100 gives a FENa of about 0.238% — well under 1%, the pattern of a kidney tightly holding onto sodium because blood flow to it has dropped.
Now a tubule that's stopped conserving much of anything: Una 60, Pcr 3.0, Pna 135, Ucr 20. Top: 60 × 3.0 = 180. Bottom: 135 × 20 = 2700. Divide: 180 ÷ 2700 ≈ 0.0667, or about 6.67% — well past the range where damaged tubule cells can no longer hold onto filtered sodium, the pattern of acute tubular necrosis.
Questions
What FENa suggests reduced blood flow to the kidney?
Under 1% is the classic prerenal reading, from Espinel's original 1976 description — the tubules are intact and aggressively reclaiming filtered sodium because something upstream, low blood volume or pressure, has cut flow to the kidney.
What FENa suggests tubular injury?
Espinel's original paper put the cutoff for acute tubular necrosis above 3%, based on a small study of just seventeen patients. Many textbooks since have simplified that to 'above 2%' — worth knowing both figures rather than assuming the commonly taught number is what the original study actually found.
Why doesn't FENa work well on diuretics?
Diuretics force the kidney to excrete sodium regardless of whether blood flow is actually reduced, so a patient with true prerenal physiology can still show a misleadingly high reading. This single limitation is what later motivated fractional excretion of urea as an alternative test.
What should be used instead of FENa in a patient on diuretics?
Fractional excretion of urea, proposed specifically for that situation because urea handling is comparatively less disturbed by diuretic use than sodium handling is.
Why does the formula involve creatinine at all?
Creatinine isn't meaningfully reabsorbed or secreted by healthy tubules, so the urine-to-plasma creatinine ratio reflects how concentrated the sample has become through water reabsorption alone. Dividing sodium excretion by that same ratio removes the effect of urine concentration, isolating sodium handling specifically.
Is a FENa in the gray zone meaningless?
Not meaningless, just less decisive. A reading between roughly 1% and 3% doesn't cleanly match either pattern Espinel described, and should be weighed alongside urinalysis findings, the clinical history, and the trend over time rather than read as a verdict on its own.
References
- Espinel CH. 1976, JAMA — original description (PubMed)
- Acute Renal Tubular Necrosis — StatPearls (NCBI Bookshelf)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.