When the kidneys fail acutely, the first diagnostic fork is whether they are under-perfused (prerenal — starved of blood flow but intact) or injured themselves (acute tubular injury). The fractional excretion of sodium interrogates exactly that: how much of the filtered sodium the tubules are letting escape. Healthy, under-perfused kidneys hoard sodium desperately; damaged tubules cannot. This FENa calculator computes the percentage from urine and plasma values and reads it against the classic thresholds.
The FENa Formula
FENa = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100. The logic inside it: creatinine clearance stands in for the filtered load, so the formula compares the fraction of filtered sodium that appears in urine against the fraction of filtered water (tracked by creatinine concentration) — a clearance ratio. Using spot urine and blood drawn at the same moment, it cancels urine flow rate entirely, which is why no timed collection is needed. One unit discipline matters: plasma and urine creatinine must share units (both mg/dL, or both µmol/L) — the sodiums, both in mmol/L or mEq/L (numerically the same), cancel the same way.
Reading the Thresholds
The textbook bands: under 1 percent → prerenal (hypovolemia, heart failure, anything starving renal perfusion, with intact tubules reabsorbing sodium maximally); over 2 percent → intrinsic tubular injury (the ATN pattern — necrotic tubules leak sodium); 1 to 2 percent is the indeterminate overlap, where either process, or a transition between them, can land. The worked examples bracket the range: urine Na 20, plasma Cr 1.0, plasma Na 140, urine Cr 100 gives (20 × 1.0) ÷ (140 × 100) × 100 = 0.14 percent, an emphatically prerenal picture; urine Na 60, plasma Cr 2.0, plasma Na 138, urine Cr 40 gives 2.17 percent, the intrinsic pattern.
The Diuretic Problem and FEUrea
Loop diuretics force sodium into the urine pharmacologically, so a prerenal patient on furosemide can post a FENa that looks intrinsic — the classic false trail. The standard workaround is the fractional excretion of urea (FEUrea), built the same way with urea in sodium’s place: urea reabsorption still tracks perfusion under diuretics, and a FEUrea under about 35 percent supports the prerenal pattern. Any FENa read in a patient who has received diuretics should carry that asterisk, which is why the interpretation under this calculator names it explicitly.
More Exceptions Worth Knowing
FENa assumes the kidney was previously sodium-competent. In chronic kidney disease, remaining nephrons chronically excrete more sodium each, pushing baseline FENa above 1 percent without any acute injury. Contrast-induced and pigment (rhabdomyolysis, hemolysis) kidney injuries can paradoxically show FENa under 1 percent despite being intrinsic — vasoconstriction dominates early. Sepsis blurs the picture further. And in glomerulonephritis the pattern is prerenal-like (avid retention) though the disease is intrinsic — the formula reads tubule behavior, not anatomy, and the wise clinician reads it that way too.
What FENa Is For — and Not For
It is one instrument in the acute kidney injury workup, alongside the history (fluid losses, blood pressure, nephrotoxic drugs), examination, urine sediment (muddy brown casts point to ATN; a bland sediment to prerenal), and the response to a fluid challenge. Its best use is confirmatory: when the story already leans prerenal and the FENa lands under 1 percent, confidence rises and the fluid trial proceeds; when the story and the number argue, the disagreement itself is the finding, and it is what sends the team back for the sediment, the bladder scan, or the ultrasound. It does not stage AKI (creatinine and urine output criteria do that), does not diagnose obstruction (ultrasound does), and never delays treatment of a clinically obvious cause. This calculator presents the computation and the standard teaching interpretation; the patient in front of a clinician always outranks the thresholds.
Common Mistakes
Mixing creatinine units between the blood (µmol/L) and urine (mg/dL) values destroys the ratio — check the lab report’s units per specimen, not per habit. Using a urine sodium from a different hour than the blood draw weakens the spot-pair logic. Reading 1.4 percent as a verdict instead of the grey zone it is, wastes the formula’s honesty. And forgetting the diuretics: a single dose of furosemide between admission and sampling is the most common reason a FENa seems to contradict an otherwise obvious prerenal story.
Related Tools
The renal corner of the Health Calculators hub also holds the MAP Calculator for the perfusion side of the story and the Shock Index Calculator for the circulation feeding the kidneys.
Frequently Asked Questions
How do you calculate FENa?
FENa = (urine sodium × plasma creatinine) / (plasma sodium × urine creatinine) × 100, using a spot urine and blood drawn at the same time. Example values 20, 1.0, 140 and 100 give 0.14 percent.
What FENa suggests a prerenal cause?
Under 1 percent. The tubules are intact and reabsorbing sodium avidly because renal perfusion is low — the pattern of volume depletion and other prerenal states.
What FENa suggests acute tubular injury?
Over 2 percent. Damaged tubules cannot reclaim filtered sodium, so the excreted fraction rises. Between 1 and 2 percent is an indeterminate zone where either process can appear.
Why do diuretics make FENa unreliable?
Loop diuretics force sodium excretion directly, so even a prerenal kidney posts a high FENa. The fractional excretion of urea (FEUrea under about 35 percent suggesting prerenal) is the standard alternative in that situation.
Do the creatinine units matter?
They must match between plasma and urine — both mg/dL or both µmol/L — because the formula is a ratio. Mixing units per specimen is the most destructive input error.
Can FENa be misleading in chronic kidney disease?
Yes. In CKD, surviving nephrons each excrete a larger sodium fraction at baseline, so FENa can exceed 1 percent without acute injury. Interpret against the patient’s baseline and the full clinical picture.
Does FENa diagnose the cause of AKI by itself?
No. It distinguishes tubular sodium handling — prerenal pattern versus intrinsic pattern — as one piece of evidence alongside history, examination, sediment and imaging. Several conditions (glomerulonephritis, early contrast or pigment injury) break the simple mapping.