Fena Calculator - AKI Pattern Bands

The FENa calculator turns four routine kidney workup labs - plasma creatinine, plasma sodium, urine creatinine, and urine sodium - into a single percentage that estimates how much filtered sodium ends up in the urine. The result is matched against the bands from the original Espinel 1976 study to suggest whether a sudden drop in kidney output is more likely a pre-renal perfusion problem (prerenal) or direct tubular damage (intrinsic, such as acute tubular necrosis). Postrenal obstruction is part of the AKI differential but is normally confirmed with renal ultrasound, not with FENa.

• • AKI pattern triage: Compare prerenal and intrinsic AKI patterns (the two conditions the FENa test was designed to separate) when urine output drops or creatinine rises.
• • Spot urine interpretation: Translate paired plasma and urine sodium and creatinine labs into a percentage that summarizes tubular sodium handling.
• • Teaching and review: Show trainees how the FENa formula links filtration to the prerenal versus intrinsic AKI bands from the original study.
• • Chart review: Check a reported FENa against a second calculation before discussing the case.

According to the National Kidney Foundation AKI overview, AKI is identified when the kidneys suddenly lose the ability to filter waste, and the FENa calculator is one of the tools clinicians typically combine with serum creatinine, urine output, urinalysis, and renal imaging to separate the three causes.

For a broader view of kidney filtration during an AKI workup, the GFR Calculator estimates glomerular filtration rate from serum creatinine and demographics so the calculation and the GFR estimate can be read side by side.

The FENa calculator uses the published formula to convert four laboratory values into a single percentage. Each input is validated before the math runs, and the percentage is matched against the prerenal, indeterminate, and intrinsic bands from the original Espinel study.

• Plasma Creatinine (PCr): Serum creatinine in mg/dL. Higher plasma creatinine with a low urine creatinine signals concentrated urine.
• Plasma Sodium (PNa): Serum sodium in mEq/L from the same draw. It is mostly constant and acts as a normalization factor.
• Urine Creatinine (UCr): Urine creatinine in mg/dL. A low value usually means a dilute spot sample and can exaggerate the FENa ratio.
• Urine Sodium (UNa): Urine sodium in mEq/L. Low urine sodium fits prerenal physiology; high urine sodium fits intrinsic AKI physiology once diuretics and dilution are ruled out.

The formula is a clearance ratio. Multiplying plasma creatinine by urine sodium captures how much sodium the kidney lets through; the denominator normalizes against urine concentration; and the factor of 100 converts the ratio to a percentage.

Interpretation is a band assignment against the Espinel 1976 thresholds. Below 1% is the prerenal band, 1% to 2% is an indeterminate gray zone, and above 2% fits intrinsic AKI such as acute tubular necrosis. Postrenal obstruction is normally confirmed with renal ultrasound.

Some clinicians also use FEUrea as a complement because FENa can be misleading on diuretics. Note FEUrea in the chart when the result is reported for a patient on diuretic therapy.

According to Espinel CH, JAMA 1976 (PMID 947239), patients with prerenal azotemia had an FENa below 1% and patients with acute tubular necrosis had an FENa above 3% (P less than .001), establishing the test as a practical way to distinguish prerenal from intrinsic AKI.

When the same basic or comprehensive metabolic panel is being reviewed alongside the calculation, the Anion Gap Calculator turns the electrolytes into a serum anion gap so the sodium and bicarbonate context can be read together.

Four ideas shape how the FENa calculator result should be read.

A correct result still requires a clean paired sample, no recent diuretic dose, and a working knowledge of whether the problem is chronic or acute. The calculator makes the math transparent but cannot replace a clinical exam, urinalysis, kidney imaging, or a discussion with the on-call nephrology team.

The same paired-sample idea shows up in proteinuria work, where the Protein Cr Ratio Calculator expresses urine protein as a ratio to urine creatinine and complements the calculation when the AKI workup also needs protein handling context.

The FENa calculator is fastest when the labs are in front of you. Enter each value once, read the percentage, then read the band label.

1. 1 Enter plasma creatinine: Use the serum creatinine in mg/dL from the same draw. If the lab reports creatinine in micromol/L, convert it first.
2. 2 Enter plasma sodium: Use the serum sodium in mEq/L from the same blood draw.
3. 3 Enter urine creatinine: Use the urine creatinine in mg/dL from a spot or timed collection taken around the same time as the blood draw.
4. 4 Enter urine sodium: Use the urine sodium in mEq/L from the same sample. A value of 0 gives an FENa of zero and the prerenal band.
5. 5 Read the FENa percentage and band: Read the percentage first, then the band label (prerenal, indeterminate, or intrinsic AKI). The diuretic note is a reminder to interpret the band with caution on diuretics.
6. 6 Document context for the clinician: Write the band, the percentage, the four input values, and the time of the draw. If the patient is on diuretics, mention FEUrea so the team can re-check the band.

A 62-year-old on the medical ward has plasma creatinine 2.0 mg/dL, plasma sodium 140 mEq/L, urine creatinine 100 mg/dL, and urine sodium 20 mEq/L from a paired draw. The result is 0.29% with a prerenal pattern band. The team then reviews volume status and blood pressure.

When the AKI workup is being done for a patient with a known stone or obstruction concern, the Kidney Stone Calculator organizes stone size, location, and symptoms so the result and the obstruction context can be reviewed together.

This FENa calculator is meant to make a routine AKI workup faster and easier to read. It does the math and the band assignment consistently so the result can be paired with the rest of the AKI workup.

• • Transparent math: Every input, the formula, and the percentage are visible on the page. The result can be cross-checked against a hand calculation.
• • Built-in band assignment: The prerenal (<1%), indeterminate (1-2%), and intrinsic AKI (>2%) bands are stated up front, so the result reads as a short triage note.
• • Diuretic reminder: Flags diuretic-influenced urine sodium values and reminds the reader to consider FEUrea.
• • Pairs with kidney workup tools: Use alongside the GFR Calculator and the Protein Cr Ratio Calculator for a more complete picture of filtration, sodium handling, and protein handling.

The calculator is also useful in teaching. A trainee can enter the same labs twice with different urine sodium values to see how the band shifts, which makes prerenal versus intrinsic physiology easier to grasp.

It is not a substitute for imaging, urinalysis, urine microscopy, or a clinical exam. The full AKI workup can include renal ultrasound, FEUrea, serology, drug review, and fluid balance review.

After the acute AKI workup has been documented, the Daily Water Intake Calculator helps frame baseline hydration patterns for the prevention discussion that often follows a review.

Several factors can move the result away from the true underlying physiology.

• • FENa is not a stand-alone diagnostic test. The percentage must be combined with urine microscopy, urine output history, blood pressure trend, medication review, and often renal imaging before a clinical decision is made.
• • FENa is not validated for chronic kidney disease, glomerulonephritis, or recent contrast dye exposure. In those settings, FEUrea or alternative tests should be considered.

The result panel warns the reader when urine sodium looks diuretic-influenced, but it is a soft warning. The clinician still has to check the medication list and the timing of the most recent diuretic dose before accepting the band label.

The wider kidney workup that follows an FENa result includes urine output, urinalysis, urine microscopy, renal imaging, and a medication review, and that broader picture drives the next clinical step.

According to NIDDK kidney disease overview, the kidneys filter extra water and wastes out of the blood and make urine, and kidney disease can develop as acute injury, chronic disease, infection, stones, or cysts.