Fractional Excretion of Sodium (FENa) Calculator

1. Read sodium and creatinine from paired urine and serum specimens.
2. Normalize urine creatinine and serum creatinine independently to mg/dL.
3. Form the filtered-to-excreted sodium ratio used by the published equation.
4. Multiply the ratio by 100 to express FENa as a percentage.
5. Retain the full-precision result until final display formatting.
6. Show the FENa percentage and normalized creatinine audit values.

The calculator does not read baseline or serial creatinine, urine-output history, diuretic exposure, CKD, urinalysis, microscopy, hemodynamics, obstruction imaging, diagnosis, or treatment information.

FENa is indirect arithmetic for the fraction of the filtered sodium load that appears in urine, using creatinine ratios to approximate the filtration relationship. It is not a direct tubular-injury measurement, GFR, eGFR, creatinine clearance, or total daily sodium excretion. A spot result describes one sampling time point.[1, 7]

Urine and serum sodium and creatinine should represent the same clinical time point as closely as practical. Fluids, diuretics, hemodynamics, and changing kidney function can alter the four inputs. Combining different times can create a ratio that described no single assessment. Clearing confirmation and stale output after any edit or unit change is a software safety measure.[4, 7]

The original report evaluated a highly selected oliguric acute renal failure population. In that study context, prerenal cases were below 1% and acute tubular necrosis cases were above 3%. CKD, diuretic exposure, nonoliguric presentations, and complex mixed causes common in modern practice were not represented as a universal validation population. The historical observations cannot be applied automatically to every person with possible AKI.[1, 6]

A prospective comparison of 87 acute renal failure patients evaluated FENa, urine sodium, urine osmolality, and the urine/plasma creatinine ratio. A 1% value separated that study population into two groups, but acute glomerulonephritis could be below 1%, while obstruction, ATN, and nonoliguric ATN could be above 1%. The finding shows that 1% is not a simple synonym for prerenal versus intrinsic disease.[2]

A 2022 systematic review included 19 studies and 1,287 patients. Among 15 studies using a 1% threshold, pooled sensitivity was 90% and specificity 82%. In studies including CKD or diuretics they were 83% and 66%; among diuretic recipients, 80% and 54%. In oliguric patients without CKD or diuretics, pooled sensitivity and specificity were 95% and 91%. These are study-level performance estimates, not a patient-level probability, and this page does not turn them into dynamic labels.[3]

FENa performed best in oliguric study populations. With preserved urine flow or nonoliguric states, sodium intake and filtered load can alter the meaning of a spot result. This page has no complete timed urine-output series, so FENa alone cannot establish or exclude oliguria.[3, 7]

CKD can change the fractional sodium excretion needed to maintain sodium balance. Diuretics can raise urine sodium and therefore raise FENa even when another clinical process is present. This page has no CKD stage, medicine timing, or last-dose data and applies no automatic correction; a higher value does not prove intrinsic injury.[3, 4, 6]

Low values have been reported in settings including glomerulonephritis or vasculitis with retained tubular sodium handling, contrast-associated vasoconstriction, pigment-related injury, early obstruction, heterogeneous or septic AKI, and strong sodium-retaining states such as cirrhosis. These examples are interpretive limitations, not diagnoses generated by this page.[6, 7]

Higher values may accompany diuretics, CKD, tubular salt wasting, sodium- or bicarbonate-associated natriuresis, treatment already given, or specimen timing. The calculator therefore does not equate a high result with ATN or another cause.[3, 4, 6]

Urine sodium is a concentration. FENa combines urine and serum sodium with urine and serum creatinine, so the two measures are not interchangeable. The 1980 study compared them in one selected cohort; it does not establish that FENa always outperforms urine sodium in every modern population. This page supplies no separate urine-sodium cutoff.[2]

FEUrea uses a different equation and has been proposed as being less affected by diuretics. Pépin 2007 did not establish it as a reliable automatic substitute in diuretic-treated patients. The 2024 Abdelhafez meta-analysis included 11 studies and 1,108 hospitalized patients; for FEUrea above 35%, pooled sensitivity was 66% and specificity 75%, with limited overall utility and no demonstrated superiority over FENa even among patients receiving diuretics.

The 2026 Pan meta-analysis used the term FEUN and included 12 studies and 1,240 patients. Overall pooled sensitivity was 0.74, specificity 0.78, and SROC AUC 0.83. In the diuretic-only subgroup, sensitivity was 0.70, specificity 0.87, and AUC 0.86. Overall heterogeneity was high (I² 90% for sensitivity and 93% for specificity), and GRADE certainty was low. The authors described moderate diagnostic accuracy and a possible adjunct role, not a stand-alone diagnosis or universal replacement rule. FEUN and FEUrea refer here to the same fractional-excretion-of-urea-nitrogen arithmetic; QuickMedCalc's FEUrea implementation converts whole urea to BUN before calculation.

The 2024 and 2026 reviews used different study sets, inclusion methods, diagnostic directions, and statistical models, so their pooled results must not be mechanically averaged. The linked PRO, CON, and commentary articles document a divided expert discussion rather than one certain rule. Together, the evidence supports FEUrea/FEUN only as possible adjunct information in selected contexts. It is not an automatic FENa replacement, a fixed 35% cause rule, a diagnostic result, or a treatment selector. This page never recommends switching tests from a submitted FENa result.[4, 5, 6, 7, 8, 9]

• AKI diagnosis or KDIGO stage.
• Prerenal, intrinsic, ATN, GN, vasculitis, or obstruction cause.
• Volume status or tubular-injury severity.
• Whether fluids or diuretics are needed.
• Urine microscopy findings or kidney recovery.
• Dialysis risk, imaging, referral, or admission.
• Medicine selection, FEUrea, or individual treatment.

AKI diagnosis and staging use clinical criteria such as serum-creatinine change and urine output, not a FENa threshold. This calculator neither changes those criteria nor produces a KDIGO diagnosis.[10]