FENa Calculator

Calculate Fractional Excretion of Sodium (FENa) and Fractional Excretion of Urea (FEUrea) to help differentiate pre-renal AKI from intrinsic renal failure. Essential for acute kidney injury workup.

Important Clinical Disclaimer: The FENa Calculator is intended for use by qualified healthcare professionals only. It is for educational and informational purposes and should not replace clinical judgement. Always interpret results alongside the full clinical picture and follow your local guidelines and protocols.
Laboratory Values
Serum Values
Normal: 135-145 mmol/L
UK standard units
Optional - required for FEUrea calculation
Urine Values (spot sample)
Spot urine sample
UK standard units
Optional - required for FEUrea calculation
Clinical Context
About FENa

FENa (Fractional Excretion of Sodium) measures the percentage of filtered sodium that is excreted in the urine.

Clinical utility:

  • Helps differentiate pre-renal AKI from intrinsic renal failure
  • Pre-renal: kidneys retain sodium avidly (FENa <1%)
  • Intrinsic: tubular damage impairs sodium reabsorption (FENa >2%)
FENa requires a spot urine sample collected at the same time as serum samples. Timed collections are not required.
FENa Interpretation
FENa Interpretation
<1% Pre-renal azotaemia
(hypovolaemia, low cardiac output, hepatorenal)
1-2% Indeterminate
(may be either pre-renal or intrinsic)
>2% Intrinsic renal disease
(ATN, interstitial nephritis, glomerulonephritis)
FEUrea Interpretation

Use FEUrea when patient is on diuretics (which invalidate FENa):

FEUrea Interpretation
<35% Suggests pre-renal AKI
35-50% Indeterminate
>50% Suggests intrinsic renal disease
Limitations

FENa may be unreliable in:

  • Diuretic use - increases FENa falsely (use FEUrea)
  • Chronic kidney disease - impaired sodium handling
  • Contrast nephropathy - may have low FENa
  • Rhabdomyolysis - FENa often <1%
  • Acute glomerulonephritis - variable FENa
  • Acute urinary obstruction - early FENa <1%
  • Sepsis - may have low FENa despite ATN
Always interpret in clinical context. FENa is one piece of the diagnostic puzzle, not definitive.

Causes of AKI by FENa Category

FENa <1% (Pre-renal)

Kidneys functioning normally, conserving sodium due to reduced perfusion:

  • Hypovolaemia:
    • Dehydration, vomiting, diarrhoea
    • Haemorrhage
    • Third-spacing (pancreatitis, burns)
  • Reduced cardiac output:
    • Heart failure
    • Cardiogenic shock
  • Systemic vasodilation:
    • Sepsis (early)
    • Cirrhosis (hepatorenal syndrome)
  • Renal vasoconstriction:
    • NSAIDs, ACEi/ARBs
    • Hypercalcaemia
FENa >2% (Intrinsic Renal)

Tubular damage impairing sodium reabsorption:

  • Acute Tubular Necrosis (ATN):
    • Prolonged ischaemia/hypoperfusion
    • Nephrotoxins (aminoglycosides, contrast)
    • Pigment nephropathy (rhabdomyolysis, haemolysis)
  • Acute Interstitial Nephritis:
    • Drug-induced (NSAIDs, antibiotics, PPIs)
    • Infection-related
  • Glomerulonephritis:
    • Post-infectious GN
    • Rapidly progressive GN
  • Vascular:
    • Vasculitis
    • Thrombotic microangiopathy

Frequently Asked Questions

Collect serum and spot urine samples simultaneously (or as close together as possible):

  1. Serum sample: Standard venous blood sample for sodium and creatinine (and urea if calculating FEUrea)
  2. Urine sample: Single voided specimen (spot urine) - no timed collection needed
  3. Request urine sodium, urine creatinine (and urine urea if needed)
  4. Ideally collect before starting diuretics or IV fluids

The samples should reflect the same physiological state, so collect them at the same time.

No - use FEUrea instead.

Diuretics (especially loop diuretics like furosemide) block sodium reabsorption in the tubules, causing sodium wasting even when the patient is volume depleted. This results in a falsely elevated FENa.

FEUrea is less affected by diuretics because urea is primarily reabsorbed in the proximal tubule and collecting duct, not the loop of Henle where furosemide acts.

  • FEUrea <35% suggests pre-renal AKI (even on diuretics)
  • FEUrea >50% suggests intrinsic renal disease

Interpretation is more difficult in CKD patients.

Patients with chronic kidney disease have impaired sodium handling at baseline:

  • Baseline FENa may be higher due to compensatory sodium excretion
  • They may not be able to retain sodium as effectively when hypovolaemic
  • A "pre-renal" FENa threshold may be higher (some suggest <2% rather than <1%)

Use FENa cautiously in CKD and interpret alongside clinical assessment of volume status, urine output trends, and response to fluid challenge.

Urine sodium alone can be helpful but FENa is more accurate.

Simple urine sodium interpretation:

  • Urine Na <20 mmol/L: Suggests pre-renal (sodium avid state)
  • Urine Na >40 mmol/L: Suggests intrinsic renal or salt wasting

Limitations of urine Na alone:

  • Affected by concentration/dilution of urine
  • Doesn't account for creatinine (GFR marker)
  • Same limitations with diuretics

FENa corrects for urine concentration by including creatinine, making it more reliable.

Several intrinsic renal conditions can present with FENa <1%:

  • Contrast nephropathy: Renal vasoconstriction preserves sodium handling early
  • Rhabdomyolysis: Myoglobin-induced AKI often has FENa <1%
  • Acute glomerulonephritis: Glomerular damage with preserved tubular function
  • Early ATN: Before significant tubular damage occurs
  • Acute urinary obstruction: Early obstruction may have low FENa
  • Sepsis: Renal vasoconstriction may maintain low FENa despite ATN

Key point: FENa <1% does not guarantee pre-renal aetiology. Always interpret in clinical context.

Disclaimer

This calculator is provided for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment.

  • Always seek the advice of a qualified healthcare provider with any questions regarding a medical condition.
  • Never disregard professional medical advice or delay seeking it because of information from this tool.
  • Clinical decision-making should always incorporate the full clinical context, patient preferences, and local protocols.
  • The creators of this tool accept no liability for decisions made based on its output.

If you are a patient: please discuss any results with your healthcare provider. This tool is designed for use by medical professionals and may not be appropriate for self-assessment.

References

  1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581.
  2. Carvounis CP, et al. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223-2229.
  3. Perazella MA, Coca SG. Traditional urinary biomarkers in the assessment of hospital-acquired AKI. Clin J Am Soc Nephrol. 2012;7(1):167-174.
  4. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2(1):1-138. https://kdigo.org/guidelines/acute-kidney-injury/
  5. NICE. Acute kidney injury: prevention, detection and management (NG148). 2019. https://www.nice.org.uk/guidance/ng148
  6. Diskin CJ, et al. The importance of timing of the urinary sodium concentration and osmolality in distinguishing pre-renal azotemia from acute tubular necrosis. Ren Fail. 2014;36(2):199-205.
  7. Bagshaw SM, et al. Urinary biomarkers in septic acute kidney injury. Intensive Care Med. 2007;33(7):1285-1296.