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Low FENa in AKI: Sodium Retention Is Not a Fluid Prescription

Quick Medical Calculator Editorial Team · Reading label 13 September 2026 · 3 min read

Retrospective study sequence, not publication date

Fractional excretion of sodium estimates the fraction of filtered sodium excreted in urine. Its practical value is greatest when the sampling conditions and clinical question are clear. It does not divide every episode of acute kidney injury into two reliably diagnosed categories.

Reproduce the Calculation Before Interpreting It

Use contemporaneous blood and urine measurements:

FENa (%) = (urine sodium * plasma creatinine) / (plasma sodium * urine creatinine) * 100

Urine and plasma sodium must use matching units, as must urine and plasma creatinine. Creatinine in mg/dL in one sample and micromol/L in the other will produce an invalid result.

In a fictional case, urine sodium is 20 mmol/L, plasma sodium 140 mmol/L, plasma creatinine 2 mg/dL, and urine creatinine 100 mg/dL:

FENa = (20 * 2) / (140 * 100) * 100 = 0.286%

This demonstrates low fractional sodium excretion. The original FENa study evaluated selected patients with acute oliguria; it did not establish that every result below 1% identifies dehydration or rules out intrinsic kidney disease. Espinel's original study.

The Interpretation Error

Suppose the same patient has breathlessness and clinical evidence of congestion. "FENa below 1%, therefore give fluid" skips the essential assessment of circulation and volume status. The index describes sodium handling, not whether additional fluid will help or harm.

Check the timing of diuretics and fluids, the creatinine trajectory, urine output, urine sediment, and the possibility of obstruction. KDIGO places laboratory findings within a broader evaluation of AKI cause, including history, examination, and imaging when indicated. Treat suspected dangerous complications promptly; do not wait for a fractional-excretion result. KDIGO AKI guideline, diagnostic approach.

FEUrea Is Not an Automatic Rescue Test

Fractional excretion of urea is sometimes requested when recent diuretic exposure makes FENa difficult to interpret. That does not make FEUrea an unaffected reference test. Two questions need separating: can an index distinguish the study's diagnostic categories, and can treatment change the index itself?

Compare Questions Before Comparing Cutoffs

In a prospective study of 99 hospitalised patients with AKI, Pepin and colleagues compared transient with persistent AKI using clinical context and return of creatinine to baseline within seven days. Among patients receiving diuretics, FEUrea at or below 35% had 79% sensitivity but 33% specificity for transient AKI. A low result therefore occurred in many patients classified as having persistent AKI. The study was small and included patients with pre-existing chronic kidney disease. Primary comparative study.

A different observational study involved 47 intensive-care patients and defined transient AKI by resolution within three days. It reported an area under the receiver operating characteristic curve of 0.78, with a wide 95% confidence interval of 0.63-0.92, and selected a 40% cutoff in that sample. The different population and outcome definition matter; a cutoff selected in the same small sample may appear more favourable than it would on independent validation. This is not a reason to replace one universal threshold with another. Dewitte and colleagues.

A third study asked a treatment-response question rather than serving as a general AKI diagnostic-accuracy trial. In 297 people admitted with hypervolaemic heart failure, Cox and colleagues compared FEUrea before and after intravenous loop diuretics. FEUrea rose by a mean 5.6 percentage points, with substantial individual variation, and 27% crossed the study's 35% classification boundary. Only a subgroup had AKI. The findings challenge the assumption that FEUrea is unchanged by loop diuresis; they do not supply an adjustment formula for all patients with AKI. Cox and colleagues.

Fictional Scenario A: A Low Result With a Congested Examination

An invented patient with a rising creatinine has breathlessness, clinical congestion and recent loop-diuretic treatment. Paired samples contain blood urea 10 mmol/L, urine urea 100 mmol/L, blood creatinine 180 micromol/L and urine creatinine 6,000 micromol/L.

FEUrea = 100 x (100/10) x (180/6000) = 30%

The arithmetic is reproducible, but “30%, therefore dehydration” is not. The result does not establish a fluid-responsive state or override the congested examination. Relevant uncertainties include the timing and response to diuresis, baseline kidney function, creatinine trajectory, urine findings and alternative causes of AKI. These fictional data do not select a treatment.

Fictional Scenario B: The Same Percentage After Gastrointestinal Losses

A different invented patient has a rising creatinine after gastrointestinal losses. Paired values of blood urea 15 mmol/L, urine urea 150 mmol/L, blood creatinine 240 micromol/L and urine creatinine 8,000 micromol/L also give:

100 x (150/15) x (240/8000) = 30%

Here the history raises different clinical possibilities, but the percentage still does not prove the cause or exclude coexisting kidney injury. There is no documented response to treatment in this example. It would be circular to assume recovery, label the AKI transient and then cite that label as confirmation that the initial FEUrea diagnosed it.

Original Sampling-Context Addendum

A useful addendum records the paired blood and urine collection times; the analytes and units; relevant diuretic and fluid exposure; the clinical volume assessment; the baseline and current creatinine; and what competing explanation remains unresolved. The two fictional scenarios demonstrate why the context belongs in the record rather than being replaced by a threshold label.

For a calculation check, blood and urine urea conventions must match, as must creatinine units. FENa measures a different fractional excretion; the percentages are not interchangeable or independent votes in a majority decision. Quick Medical Calculator's FEUrea display places exactly 35% in its upper category, whereas Pepin included 35% in the low category. State whether the software convention or the study convention is being used before comparing an exact-35% result. A software label is not stronger evidence than the source population and sampling conditions support.

Record what the measurement adds and what it cannot determine. Urgent assessment and management of dangerous AKI complications must not wait for either index. Neither a low FEUrea nor agreement between FEUrea and FENa establishes safety or determines a fluid prescription.

A Better Chart Entry

For the worked case: "FENa 0.29% from paired samples; consistent with sodium retention. Interpret with recent treatment, hemodynamics, urine findings, and AKI trajectory; not sufficient to determine fluid responsiveness."

For deciding whether the change meets AKI criteria or represents established CKD, use the separate acute-versus-chronic kidney assessment guide. That temporal question comes before assigning a cause from urine chemistry.

Retrospective educational reading plan: Day 9 of 50, retrospective reading date 2026-09-13. This is a study-sequence label, not a publication date, review date or source-check date. The original first-publication timestamp is retained.