FENa calculator estimates fractional excretion of sodium, a percentage that shows how much filtered sodium the kidneys pass into urine. During suspected acute kidney injury, clinicians may use FENa as one clue to understand whether the kidneys are conserving sodium because of reduced blood flow or losing more sodium because of possible tubular injury.
It is not a diagnosis by itself, but it can help make kidney-related lab results easier to review when interpreted with symptoms, urine output, medications, fluid status, and other test results.
The Medicalic FENa calculator is designed to make the calculation clear, accessible, and easier to review, not to replace clinical judgment. A FENa result cannot diagnose kidney failure, identify the cause of acute kidney injury, or decide treatment by itself. Interpretation depends on symptoms, urine output, fluid status, blood pressure, medications, kidney history, and other lab results. [1]
This calculator uses four values: serum sodium, urine sodium, serum creatinine, and urine creatinine. It supports sodium in mEq/L or mmol/L and creatinine in mg/dL or µmol/L, so it can work with many U.S. and international lab reports.
What Is FENa?
FENa stands for fractional excretion of sodium. It estimates the percentage of filtered sodium that leaves the body in urine instead of being reabsorbed by the kidneys. In simple terms, it compares sodium in blood and urine while using creatinine to adjust for urine concentration.
FENa is mainly used as an interpretation aid in acute kidney injury evaluation, not as a general kidney screening test.
FENa is most often discussed during acute kidney injury evaluation. AKI causes are commonly grouped into prerenal, intrinsic renal, and postrenal categories, although real cases can overlap.[1] FENa can support clinical reasoning, but it should never be treated as a standalone kidney diagnosis.
How to Use the FENa Calculator
To use the FENa calculator, enter values from the same clinical evaluation whenever possible.
- Enter serum sodium. Choose mEq/L or mmol/L.
- Enter urine sodium. Choose mEq/L or mmol/L.
- Enter serum creatinine. Choose mg/dL or µmol/L.
- Enter urine creatinine. Choose mg/dL or µmol/L.
- Review the FENa percentage and the general interpretation.
Use blood and urine results collected close together. FENa works best when the samples reflect the same clinical moment. Avoid mixing older blood values with newer urine values unless a clinician specifically asks you to compare them.
Also check the creatinine units carefully. Serum and urine creatinine may be reported in mg/dL in the United States or µmol/L in many international reports. A calculator that accepts both units should convert them consistently, but choosing the wrong unit can produce an incorrect result.
FENa Calculator Formula
The standard FENa calculator formula is:
It may also be written as:
Where:
- UNa = urine sodium
- SNa = serum sodium
- UCr = urine creatinine
- SCr = serum creatinine
Creatinine helps adjust for urine concentration. This is why FENa can provide more context than urine sodium alone. The formula is commonly described as urine sodium multiplied by serum or plasma creatinine, divided by serum or plasma sodium multiplied by urine creatinine, then multiplied by 100. [2]
FENa Example Calculation
Imagine the lab values are:
- Serum sodium: 140 mEq/L
- Urine sodium: 20 mEq/L
- Serum creatinine: 1.2 mg/dL
- Urine creatinine: 100 mg/dL
Place those values into the formula:
A FENa of 0.17% is below 1%. That pattern is often associated with sodium conservation, which can fit a prerenal pattern. It does not prove the cause of AKI. It simply adds one clue to the broader clinical picture.
How to Interpret Your FENa Result
In many adult acute kidney injury discussions, FENa is commonly interpreted using these approximate cutoffs:
| FENa result | General pattern |
| Less than 1% | Often associated with a prerenal pattern |
| 1–2% | Indeterminate or borderline |
| More than 2% | Often associated with an intrinsic renal pattern |
A low FENa often fits sodium retention seen in prerenal states. A higher FENa can suggest tubular sodium wasting, which may occur with intrinsic kidney injury such as acute tubular injury. Intermediate values are less clear and need clinician interpretation. [3,6]
What Low, Borderline, and High FENa May Suggest
A result below 1% may suggest that the kidneys are holding onto sodium. Clinicians often see this pattern when kidney perfusion is reduced, such as with dehydration, low effective circulating volume, heart failure, cirrhosis-related fluid imbalance, or other prerenal AKI patterns.
Results between 1% and 2% are harder to interpret. Fluids, medications, early tubular injury, chronic kidney disease, and changing urine output can all push the result into a borderline range. This range usually needs trend review and clinical context.
When the result is above 2%, it often points toward an intrinsic renal pattern, especially tubular injury where the kidneys are less able to reabsorb sodium. Acute tubular necrosis is a common example discussed with higher FENa values. Postrenal obstruction can produce variable results, so FENa should not rule obstruction in or out by itself.
When FENa Is Useful
FENa may be useful when clinicians evaluate acute kidney injury and want to understand whether sodium handling fits a prerenal or intrinsic renal pattern. It is generally most informative before diuretics are given and when blood and urine samples are collected close together during an oliguric AKI evaluation. [4]
A FENa result works best as part of a broader assessment. Clinicians may also review physical examination findings, fluid balance, urine output, blood pressure, urine microscopy, kidney ultrasound, medication use, and creatinine trends. [1]
When FENa Can Be Misleading
FENa can mislead in several common situations. Diuretics are one of the biggest issues because they increase sodium excretion and may make FENa look higher. Chronic kidney disease can also reduce the usefulness of FENa. A 2022 systematic review found that FENa has a more limited role in people with CKD or diuretic therapy and is most valuable when oliguria is present. [4]
FENa may also be harder to interpret in non-oliguric AKI, sepsis, acute glomerulonephritis, contrast-associated kidney injury, rhabdomyolysis or pigment nephropathy, urinary obstruction, kidney transplant complications, neonates, and mixed AKI causes.
FENa vs Urine Sodium and FEUrea
Urine sodium alone tells you the sodium concentration in urine, but urine concentration can change with hydration and kidney handling. FENa adjusts urine sodium using serum sodium and creatinine, which gives a fractional measure instead of a simple concentration.
FEUrea, or fractional excretion of urea, is sometimes considered when diuretics may interfere with FENa because diuretics directly alter sodium excretion. FEUrea is not perfect either. Reviews describe FENa and FEUrea as useful tools in selected settings, but both require clinical context. [5]
Common Mistakes to Avoid
Common mistakes include entering serum sodium where urine sodium belongs, reversing serum and urine creatinine, choosing the wrong creatinine unit, or using blood and urine values from different times. Another mistake is interpreting FENa without considering diuretics, CKD, sepsis, obstruction, recent IV fluids, or urine output.
Adult cutoffs should also be used carefully in children and neonates. Pediatric interpretation can differ, so a qualified clinician should interpret results for infants and children.
FENa Calculator FAQs
Not always. Diuretics can increase sodium excretion and may make FENa misleading.
No. FENa is one clue in AKI evaluation. It cannot diagnose kidney failure or identify the cause by itself.
FEUrea may be considered when diuretics affect sodium excretion, but it also has limitations. A clinician should interpret either result with the full clinical picture.
Medical References
Update History
Originally written by Medicalic.
Your answer helps us improve the clarity and usefulness of our health content.