Enter paired serum and urine values
Use the same sampling period whenever possible. Sodium is typically entered as mEq/L or mmol/L. For sodium, those units are numerically equivalent here.
Example interpretation table
| Scenario | Serum Na | Urine Na | Serum Cr | Urine Cr | FENa | Pattern note |
|---|---|---|---|---|---|---|
| Likely prerenal physiology | 140 | 10 | 2.0 mg/dL | 200 mg/dL | 0.07% | Strong sodium retention pattern |
| Low FENa example | 138 | 20 | 1.8 mg/dL | 150 mg/dL | 0.17% | Often supports prerenal state |
| Borderline range | 136 | 60 | 2.2 mg/dL | 180 mg/dL | 0.54% | Needs broader context review |
| Intrinsic pattern example | 140 | 80 | 2.5 mg/dL | 60 mg/dL | 2.38% | Intrinsic tubular injury more likely |
These rows are educational examples. Real interpretation depends on timing, medications, urine microscopy, comorbid disease, and overall kidney injury context.
How the calculation works
This equation estimates the percentage of filtered sodium that ends up excreted in urine. Sodium alone can be misleading because urine concentration changes with water handling. Creatinine is included to normalize the sodium measurements against filtration and concentration effects.
Urine Sodium ÷ Serum Sodium
Urine Creatinine ÷ Serum Creatinine
If you enter creatinine in µmol/L, the page first converts it to mg/dL using the relation 1 mg/dL = 88.4 µmol/L. That keeps the serum and urine creatinine terms internally consistent before calculating FENa.
Steps for using this calculator
- Enter a case label if you want a named export report.
- Type the serum sodium and urine sodium values from paired samples.
- Enter serum and urine creatinine values, then choose the correct creatinine units.
- Mark recent diuretic use when relevant, because that affects interpretation reliability.
- Add optional notes for context, such as hypotension, oliguria, or medication timing.
- Press Calculate FENa to view the result above the form.
- Review the interpretation banner, converted values, and the Plotly graph.
- Use the CSV or PDF buttons to save the result summary.
Frequently asked questions
1) What is FENa?
FENa is the percentage of filtered sodium excreted in urine. It compares urine sodium and creatinine with serum sodium and creatinine. Clinicians use it to help distinguish sodium-avid states from intrinsic tubular injury patterns during acute kidney evaluation.
2) When is FENa most useful?
FENa is most useful in acute kidney injury when paired blood and urine samples are obtained close together. It adds context to volume assessment, medication history, urine microscopy, and hemodynamics rather than replacing them.
3) What does a FENa below 1% usually suggest?
Values below 1% often suggest prerenal physiology because the kidneys are retaining sodium. However, sepsis, contrast exposure, pigment injury, chronic kidney disease, and timing of collection can change the picture.
4) What does a FENa above 2% usually suggest?
Values above 2% often suggest intrinsic tubular injury, especially acute tubular necrosis. Still, the number is supportive, not definitive, and should be interpreted with symptoms, exam findings, urinalysis, and trends.
5) How do diuretics affect FENa?
Diuretics can increase urinary sodium and falsely raise FENa. In that setting, some clinicians also review FEUrea, urine microscopy, and the patient’s recent medication timeline before drawing conclusions.
6) Do units matter?
Sodium should be entered in matching concentration units for serum and urine, usually mEq/L or mmol/L. Creatinine should also be matched, and this page can convert creatinine between mg/dL and µmol/L.
7) Can this calculator diagnose acute kidney injury?
No. This calculator is an educational support tool. Acute kidney injury diagnosis and management require history, examination, urine output, labs, imaging when appropriate, and professional clinical judgment.
8) Why is creatinine included in the formula?
Creatinine helps normalize sodium concentrations for filtration and water handling. Using both sodium and creatinine from serum and urine makes the estimate more robust than urine sodium alone.