The short answer: Fractional excretion of sodium (FENa) is a urine-based calculation that estimates how much of the sodium filtered by the kidneys is ultimately excreted in urine. Traditionally, a FENa below 1% has suggested a prerenal cause of acute kidney injury (AKI), while a FENa above 2% has suggested intrinsic kidney injury, particularly acute tubular injury.
But this rule is not definitive. Current evidence shows that FENa works best in selected patients, especially those with oliguria who are not taking diuretics and do not have established CKD. In many real-world cases, medications, CKD, sepsis, contrast exposure, and recent fluid treatment can make FENa difficult to interpret. (KDIGO)
What Is FENa?
FENa stands for fractional excretion of sodium.
It estimates the percentage of filtered sodium that the kidneys actually excrete into the urine.
The calculation is:
FENa (%) = [(Urine sodium × Serum creatinine) ÷ (Serum sodium × Urine creatinine)] × 100
In simple terms, FENa helps answer:
Are the kidneys strongly holding on to sodium, or are they allowing more sodium to leave in the urine?
When blood flow to the kidneys falls but the kidney tubules are still functioning appropriately, the kidneys tend to retain sodium and water. This can produce a low FENa.
With some forms of intrinsic kidney injury, particularly acute tubular injury, the tubules may have difficulty reabsorbing sodium, resulting in a higher FENa.
How Does FENa Help With AKI?
Acute kidney injury means a relatively sudden decline in kidney function. AKI can have several causes, broadly grouped as:
- Prerenal: reduced blood flow or effective circulation to the kidneys
- Intrinsic renal: injury within the kidney itself
- Postrenal: obstruction of urine flow
FENa is mainly used to help distinguish prerenal AKI from certain forms of intrinsic AKI.
It does not reliably identify every cause of AKI, and it does not replace the rest of the diagnostic evaluation.
FENa Values: The Traditional Interpretation
| FENa | Traditional interpretation |
|---|---|
| <1% | Supports a prerenal pattern |
| 1–2% | Indeterminate; requires clinical context |
| >2% | Supports intrinsic tubular injury in the appropriate setting |
These cutoffs are useful teaching points, but they should not be treated as diagnostic laws.
Older clinical literature commonly used <1% for prerenal AKI and >2% for intrinsic AKI. (AAFP)
More recent evidence shows that FENa has substantially lower accuracy when patients have CKD, are receiving diuretics, or have already received fluids or vasoactive medications. The 2026 KDIGO AKI/AKD guideline public-review draft therefore recommends reserving FENa and related urine chemistries for selected clinical situations, rather than using them routinely for every patient with AKI. (KDIGO)
Why Is FENa Low in Prerenal AKI?
Imagine someone becomes significantly dehydrated from vomiting, diarrhea, bleeding, or inadequate fluid intake.
Blood circulation reaching the kidneys may fall.
The kidneys respond by trying to conserve both sodium and water. More sodium is reabsorbed rather than lost in urine.
As a result, the FENa may fall below 1%.
This does not necessarily mean the kidney tissue is normal in every case. It simply reflects how the kidneys are handling sodium at that particular time.
Why Can FENa Be Higher in Intrinsic AKI?
In acute tubular injury, the kidney tubules can become damaged and lose some of their ability to reabsorb sodium.
More sodium remains in the urine, so FENa may rise.
However, not every intrinsic kidney disorder produces a high FENa. Some patients with intrinsic AKI can have a FENa below 1%, particularly early in the disease or in conditions such as contrast-associated injury, rhabdomyolysis, and some forms of glomerulonephritis. (KDIGO)
This is one reason a low FENa should not automatically be translated into:
“There is no intrinsic kidney injury.”
When Can FENa Be Misleading?
Several situations can interfere with interpretation.
Diuretics
Diuretics such as furosemide increase sodium excretion.
That can make FENa appear elevated even when the underlying problem is hemodynamic or prerenal.
This is one of the most important limitations of FENa. (AAFP)
Chronic Kidney Disease
People with CKD may handle sodium differently from people with previously normal kidney function.
As a result, traditional FENa thresholds may be less reliable.
Sepsis and Critical Illness
AKI associated with sepsis and other severe illnesses can involve several overlapping mechanisms. A single FENa measurement may not cleanly separate them.
Recent Fluids or Medications
FENa reflects what the kidneys are doing at the time of the urine and blood samples.
Recent IV fluids, diuretics, vasopressors, and other treatments can change urine sodium handling and complicate interpretation. (KDIGO)
Early or Atypical Intrinsic AKI
Some intrinsic kidney disorders can produce a low FENa, meaning that FENa <1% does not prove prerenal AKI.
What About FEUrea?
Fractional excretion of urea (FEUrea) is another urine-based calculation sometimes used when FENa is difficult to interpret.
Traditionally, a FEUrea below about 35% has been considered supportive of a prerenal pattern.
It is sometimes discussed as an alternative in patients taking diuretics because diuretics directly increase sodium excretion. However, newer evidence also shows that FEUrea has important limitations and should not simply be considered a universally reliable replacement for FENa. (KDIGO)
FENa Is Only One Piece of the AKI Evaluation
A good AKI assessment usually considers several pieces of information together.
1. The clinical history
Ask what happened before the creatinine increased:
- Vomiting or diarrhea?
- Poor fluid intake?
- Bleeding?
- Infection or sepsis?
- Low blood pressure?
- Recent surgery?
- New medications?
- NSAID use?
- Recent contrast exposure?
2. The creatinine trend
One creatinine value cannot always tell you whether AKI is improving or worsening.
The useful sequence is:
Previous creatinine → current creatinine → size of change → time interval → clinical context
3. Urine examination
Urinalysis and urine microscopy can provide important clues.
For example, certain casts or blood/protein patterns may suggest intrinsic kidney disease rather than uncomplicated prerenal AKI.
4. Urine output
Reduced urine production (oliguria) can provide important information, although AKI can also occur with relatively normal urine output.
5. Imaging when appropriate
If urinary obstruction is possible, kidney imaging—often ultrasound—is used to evaluate for a postrenal cause.
This matters because FENa is primarily intended to help with the prerenal-versus-intrinsic question; it does not rule out obstruction. (AAFP)
Does a Low FENa Mean the Kidneys Are Fine?
No.
A low FENa means the kidneys are retaining sodium relative to the amount filtered. That pattern can occur with prerenal AKI, but it can also occur in some intrinsic kidney disorders.
Similarly, a high FENa does not automatically prove irreversible kidney damage.
AKI can result from different mechanisms, and kidney function can sometimes recover substantially after the underlying problem is treated.
What Should You Do With a FENa Result?
If FENa appears on a medical report, avoid interpreting it by itself.
A clinician will generally consider it alongside:
- Serum creatinine and its trend
- Urine output
- Blood pressure and volume status
- Urinalysis and urine microscopy
- Medication exposure
- Diuretic use
- CKD history
- Recent fluids or other treatment
- Electrolytes and acid-base status
- Possible urinary obstruction
The cause of AKI matters because treatment depends on the cause. For example, managing dehydration is very different from treating obstruction, glomerular disease, sepsis, or acute tubular injury.
When Should AKI Be Taken Urgently?
AKI deserves prompt medical assessment, particularly when kidney function is changing quickly or the person is acutely unwell.
Urgent evaluation is particularly important with:
- Very low or suddenly decreasing urine output
- Severe dehydration or persistent vomiting
- Low blood pressure or fainting
- Severe shortness of breath
- Significant swelling
- Confusion
- Chest symptoms
- Dangerous electrolyte abnormalities
- Rapidly worsening kidney-function tests
These findings do not identify the cause by themselves, but they can indicate a need for immediate assessment.
Bottom Line
FENa can help distinguish a prerenal pattern from intrinsic tubular injury, but it is not a stand-alone diagnostic test.
Traditionally, FENa <1% supports prerenal AKI and >2% supports intrinsic tubular injury. However, these thresholds become less reliable with diuretics, CKD, sepsis, recent fluid treatment, and several forms of intrinsic kidney disease. (KDIGO)
The most useful approach is to interpret FENa alongside the creatinine trend, urine output, medication history, volume status, urinalysis, and possible obstruction.
In other words:
FENa is a clue—not a diagnosis.