FENa Calculator
Use this tool to calculate the fractional excretion of sodium (FENa), commonly used in acute kidney injury workups.
FENa (%) = [(Urine Sodium × Plasma Creatinine) / (Plasma Sodium × Urine Creatinine)] × 100
Important: this calculator is educational and should not replace clinical judgment.
What Is Fractional Excretion of Sodium?
Fractional excretion of sodium (FENa) estimates the percentage of filtered sodium that is ultimately excreted in urine. In practical terms, it helps clinicians evaluate how kidneys are handling sodium during possible kidney injury.
FENa is often used when differentiating causes of acute kidney injury (AKI), especially when considering:
- Prerenal states (reduced kidney perfusion, such as dehydration or low effective blood volume)
- Intrinsic renal injury (such as acute tubular injury/necrosis)
How to Use This FENa Calculator
Step-by-step
- Enter urine sodium concentration.
- Enter plasma sodium concentration.
- Enter urine creatinine and plasma creatinine values in the same units.
- Click Calculate FENa.
The calculator will return FENa as a percentage and provide a quick interpretation.
Interpreting FENa Results
Typical teaching ranges (adults, non-diuretic context):
- < 1%: often suggests prerenal physiology (kidneys conserving sodium).
- 1% to 2%: borderline or indeterminate zone; interpret with full clinical context.
- > 2%: often suggests intrinsic tubular injury with reduced sodium reabsorption.
These thresholds are not absolute. Clinical history, urine sediment, hemodynamics, medication effects, and serial labs remain essential.
When FENa Can Be Misleading
1) Diuretic use
Loop and thiazide diuretics can increase urinary sodium excretion, making FENa appear higher than expected and reducing diagnostic reliability.
2) Chronic kidney disease (CKD)
In advanced CKD, tubular handling of sodium may already be altered, so standard FENa cutoffs may not perform as well.
3) Early or mixed AKI states
Some patients have overlapping causes (for example, sepsis with hypoperfusion and tubular injury), where a single index cannot fully classify pathology.
4) Contrast exposure and pigment nephropathy
In selected settings, FENa may stay low despite intrinsic injury, so a low value does not always exclude intrinsic renal damage.
FENa vs FEUrea
In patients on diuretics, clinicians may use fractional excretion of urea (FEUrea) as an additional clue because it can be less affected by natriuretic therapy. Even then, it should be interpreted as supportive data, not a standalone diagnosis.
Clinical Reminder
FENa is most useful as part of a broader diagnostic framework that includes exam findings, fluid status, blood pressure trends, urinalysis, microscopy, and response to treatment.
- Use trends rather than isolated single values when possible.
- Reassess after interventions (fluids, medication changes).
- Correlate with patient-specific context and comorbidities.
Quick Example
If urine sodium = 20 mEq/L, plasma sodium = 140 mEq/L, urine creatinine = 100 mg/dL, and plasma creatinine = 2.0 mg/dL:
FENa = [(20 × 2.0) / (140 × 100)] × 100 = 0.29%
This low value may support prerenal physiology in the right clinical setting.
Disclaimer
This page is for educational use and not a substitute for professional medical advice, diagnosis, or treatment.