Educational use only: FENa does not diagnose the cause of AKI by itself
Fractional excretion of sodium is a calculation, not a clinical decision system. It can support bedside reasoning in acute kidney injury, but it cannot determine whether a patient has prerenal azotemia, acute tubular necrosis, obstruction, glomerulonephritis, sepsis-associated kidney injury, hepatorenal physiology, or medication-related injury. Use it only with the full clinical picture: vital signs, volume assessment, urine output, medication timeline, urinalysis, microscopy, serum chemistry trend, hemodynamics, imaging when indicated, and local nephrology or critical care protocols.
FENa calculation at a glance
Formula: FENa (%) = (urine sodium x serum creatinine) / (serum sodium x urine creatinine) x 100.
Creatinine units matter: serum and urine creatinine must be converted to the same unit before the ratio is calculated.
Classic bands: below 1% suggests prerenal physiology, 1-2% is indeterminate, and above 2% suggests intrinsic tubular injury.
Clinical boundary: diuretics, CKD, sepsis, obstruction, contrast exposure, and mixed AKI mechanisms can make the result misleading.
# What fractional excretion of sodium measures
Fractional excretion of sodium estimates the percentage of filtered sodium that appears in the urine. In simplified terms, it compares sodium handling with creatinine handling. Creatinine is used as a filtration marker, while sodium reflects tubular reabsorption. If renal perfusion is reduced and tubular function is intact, the kidney often conserves sodium aggressively, producing a low urine sodium concentration and a low FENa. If tubular cells are injured, sodium reabsorption may be impaired, and FENa can rise.The calculation became popular because it can be performed from commonly available serum and spot urine values. It is most often discussed in acute kidney injury when clinicians are trying to distinguish a prerenal pattern from intrinsic tubular injury. The result is not a direct measurement of renal blood flow, tubular viability, or kidney prognosis. It is a physiologic clue that can be helpful when the pretest probability and sampling conditions are appropriate.- FENa
- Fractional excretion of sodium, reported as the percent of filtered sodium excreted in urine.
- Prerenal physiology
- A pattern where reduced effective kidney perfusion leads to sodium and water conservation if tubules can still respond.
- Intrinsic AKI
- Kidney injury originating within the kidney parenchyma, including acute tubular injury or necrosis.
- Spot urine
- A single urine sample used for urine sodium and creatinine rather than a timed urine collection.
- Creatinine normalization
- Using creatinine in the ratio to account for urine concentration and filtration effects.
| Input | Common units | Why it is needed | Practical check |
|---|---|---|---|
| Serum sodium | mEq/L or mmol/L | Represents filtered sodium concentration | Use a serum value near the urine sample time |
| Urine sodium | mEq/L or mmol/L | Shows sodium appearing in urine | Can be altered by diuretics or saline administration |
| Serum creatinine | mg/dL or umol/L | Part of filtration normalization | Convert to same unit as urine creatinine |
| Urine creatinine | mg/dL or umol/L | Adjusts for urine concentration | Very dilute urine can magnify uncertainty |
# How to interpret the three FENa zones
Below 1%
Classically supports prerenal physiology in an oliguric patient who has not received diuretics and has intact tubular sodium conservation.
- Think reduced effective circulating volume
- Look for bland urine sediment or hyaline casts
- Confirm with hemodynamics and clinical response
1% to 2%
An overlap zone where single-number interpretation is especially weak and mixed physiology is common.
- Repeat context matters more than the exact decimal
- Review sampling timing and recent fluids
- Use urine microscopy and trajectory
Above 2%
Often supports intrinsic tubular dysfunction, including acute tubular injury, when the clinical setting fits.
- Can occur with ATN
- May also reflect diuretics or salt wasting
- Do not ignore obstruction or glomerular disease
FENa is most fragile after diuretics
Loop and thiazide diuretics increase urinary sodium excretion by design. If urine sodium is measured after diuretic exposure, FENa may look high even when effective kidney perfusion is reduced. In that setting, clinicians may consider fractional excretion of urea, urine microscopy, hemodynamic assessment, and the medication timeline, but no single urine index fully solves the problem.
<1% classic prerenal threshold
1-2% overlap zone
>2% classic intrinsic threshold
88.4 umol/L per 1 mg/dL creatinine
# Unit conversion and common calculation mistakes
Sodium is straightforward because mEq/L and mmol/L are numerically equivalent for sodium, a monovalent ion. Creatinine is the common source of error. Some laboratories report serum creatinine in mg/dL and urine creatinine in mg/dL, while others report umol/L. FENa is a ratio, so the absolute unit chosen is less important than using the same creatinine unit in both numerator and denominator. This calculator converts creatinine internally to mg/dL before calculating.| Conversion | Formula | Example |
|---|---|---|
| Creatinine mg/dL to umol/L | mg/dL x 88.4 | 2.0 mg/dL = 176.8 umol/L |
| Creatinine umol/L to mg/dL | umol/L / 88.4 | 177 umol/L = 2.0 mg/dL |
| Sodium mEq/L to mmol/L | same numeric value | 140 mEq/L = 140 mmol/L |
| FENa percent | ratio x 100 | 0.006 ratio = 0.6% |
Use paired samples when possible
FENa is most coherent when serum and urine samples represent the same physiologic moment. A urine sample collected before fluids, vasopressors, diuretics, or contrast exposure may tell a different story than blood drawn hours later. When timestamps differ, document the sequence before treating the decimal value as meaningful.Worked example
Suppose serum sodium is 140 mEq/L, urine sodium is 20 mEq/L, serum creatinine is 2.0 mg/dL, and urine creatinine is 120 mg/dL. FENa = (20 x 2.0) / (140 x 120) x 100 = 0.24%. In the classic teaching framework, that is below 1% and supports sodium conservation. It does not prove volume depletion; it must be compared with the patient presentation and urine findings.
# When FENa is less reliable
- Recent diuretic use: urine sodium may rise independent of perfusion status, pushing FENa upward.
- Chronic kidney disease: baseline tubular handling and creatinine kinetics may not match the classic assumptions.
- Sepsis and critical illness: renal blood flow, tubular function, inflammation, vasopressors, and fluids can produce mixed physiology.
- Acute glomerulonephritis: sodium retention may occur despite intrinsic renal disease, creating a low FENa.
- Urinary obstruction: early and late phases may produce different urine sodium patterns.
- Contrast-associated injury or pigment nephropathy: urine indices may be variable and should not replace the broader AKI workup.
Strengths and limits of FENa at the bedside
Advantages
- Uses routine serum and spot urine values that are often available quickly.
- Provides a physiologic frame for sodium conservation versus sodium wasting.
- Easy to teach and transparent because the formula is a simple ratio.
- Can complement urine microscopy and hemodynamic assessment.
Disadvantages
- Accuracy depends heavily on timing, medications, and clinical selection.
- Low FENa can occur in some intrinsic kidney diseases, including glomerulonephritis.
- A single cutoff can hide overlap between prerenal and intrinsic processes.
- It cannot identify the cause of tubular injury or determine treatment.
FENa should travel with urine microscopy
Urine sediment often adds information that FENa cannot provide. Muddy brown granular casts can support acute tubular injury; red blood cell casts can suggest glomerulonephritis; white blood cell casts may point toward interstitial nephritis or pyelonephritis. A low or high FENa without sediment review is a thinner argument.
# Questions clinicians should ask before trusting FENa
The usefulness of FENa depends on whether the sample was collected under conditions that match the original clinical idea behind the test. A clean teaching scenario is an oliguric patient with new AKI, no recent diuretic exposure, no advanced chronic kidney disease, and a question of reduced effective arterial volume versus tubular injury. Real hospital patients often have several competing influences: crystalloid resuscitation, vasopressors, heart failure, cirrhosis, sepsis, antibiotics, contrast, and changing urine output. The more complex the physiology, the less a single FENa number should dominate the assessment.- Was the patient oliguric? Classic FENa teaching performs best in oliguric AKI; nonoliguric injury can produce less predictable urine indices.
- Were serum and urine samples paired? Large time gaps can mix two physiologic states, especially after fluids, diuretics, or vasopressors.
- Were diuretics given? Natriuresis from medication can make a prerenal process look intrinsically sodium-wasting.
- Is there CKD or transplant kidney disease? Baseline tubular handling and creatinine dynamics may differ from the assumptions behind the cutoff.
- Is obstruction excluded when appropriate? Postrenal AKI can produce variable sodium indices and should be addressed anatomically, not by FENa alone.
| Scenario | Possible FENa behavior | Why caution is needed |
|---|---|---|
| Vomiting or hemorrhage before diuretics | Often below 1% | Supports sodium conservation but still requires clinical confirmation |
| Acute tubular injury after prolonged shock | Often above 2% | Fits impaired tubular reabsorption but does not identify the insult |
| Glomerulonephritis | May be below 1% | Intrinsic disease can still show sodium retention |
| Diuretic-treated heart failure | Can be above 1% or above 2% | Medication effect may overwhelm perfusion physiology |
| Early obstruction | Variable | Urine electrolytes cannot replace bladder scan or imaging when suspected |
Treat discordance as a signal to slow down
If FENa conflicts with the bedside picture, do not force the diagnosis to fit the number. Recheck the timestamps, medication administration record, urine sediment, fluid balance, and whether the creatinine rise is still evolving. A discordant FENa often means the patient has mixed physiology or the sample was collected after treatment changed renal sodium handling.# How FENa fits into an AKI evaluation
Acute kidney injury evaluation starts with the clinical question: is kidney perfusion reduced, is there intrinsic renal injury, or is urine flow obstructed? FENa addresses only one part of that question: sodium handling at the nephron level. It does not replace checking urine output trend, blood pressure, volume losses, heart failure or cirrhosis physiology, nephrotoxic exposures, urinalysis, bladder scan, renal ultrasound when indicated, and serial creatinine response.A useful way to use FENa is to ask whether it agrees with the rest of the case. If the patient has hypotension, gastrointestinal fluid loss, bland sediment, no diuretic exposure, and FENa below 1%, the result supports the working model. If the patient has sepsis, vasopressors, loop diuretics, granular casts, and FENa 1.4%, the indeterminate number should not slow down broader management. Discordance is often the most educational part of the test.Clinical bottom line
FENa is best treated as a bedside physiology clue. Below 1%, 1-2%, and above 2% are useful teaching bands, but the value becomes meaningful only when paired with sampling timing, medication exposure, urine microscopy, hemodynamics, and the full AKI differential. This calculator intentionally gives cautious interpretive language because the number should support clinical reasoning, not replace it.