What are the next steps after calculating the fractional excretion of sodium (FENa)?

Clinical Interpretation and Management Based on FENa Results

After calculating FENa, use it to differentiate the cause of acute kidney injury (AKI) and guide volume management, with FENa <1% suggesting prerenal causes requiring volume repletion and FENa >1% suggesting intrinsic renal injury, though interpretation must account for clinical context including diuretic use, chronic kidney disease, and specific disease states. 1

Primary Diagnostic Algorithm

FENa <1%: Prerenal Causes

• Indicates volume depletion or sodium-avid states requiring fluid resuscitation 1, 2
• Consider prerenal azotemia, hypovolemia, hepatorenal syndrome, or heart failure 1
• In patients with acute tubular necrosis, a marked decrease in FENa suggests superimposed volume depletion that may delay recovery—treat with vigorous intravenous fluid therapy 3
• Monitor response to volume repletion with improvement in urine output and renal function 3

FENa >1%: Intrinsic Renal Injury

• Suggests structural kidney damage such as acute tubular necrosis 1
• Indicates widespread tubular dysfunction requiring supportive management rather than volume expansion 2
• In oliguric patients with acute tubulointerstitial nephritis, FENa is almost constantly elevated 4

Critical Clinical Context Modifiers

Diuretic Use

• Patients receiving diuretics can have low FENa despite intrinsic kidney injury, rendering standard interpretation unreliable 1, 2
• Consider fractional excretion of urea (FEUrea) as an alternative, with FEUrea <28.16% having 75% sensitivity and 83% specificity for hepatorenal syndrome 1

Heart Failure

• In acute decompensated heart failure, FENa >0.4% measured more than 6 hours after diuretic dosing predicts worsening renal impairment (OR=6.3) 5
• Use FENa to assess sodium avidity and guide diuretic response 1
• Patients with FENa >0.4% have longer hospitalizations and more frequent cardiac care unit admissions 5

Cirrhosis and Hepatorenal Syndrome

• Standard FENa cutoffs are less reliable due to altered sodium handling; FENa <1% has 100% sensitivity but only 14% specificity for prerenal causes 1
• Monitor sodium excretion to guide diuretic therapy in patients with ascites 1
• A spot urine sodium/potassium ratio >1 indicates sodium excretion >78 mmol/day with 90-95% confidence 1

Cystic Fibrosis

• Use FENa to assess sodium supplementation needs, maintaining target range of 0.5-1.5% 6, 1
• For routine practice, urinary sodium:creatinine ratio (corresponding range 17-52 mmol/mmol) correlates with FENa and is easier to measure 6

Important Pitfalls to Avoid

Non-Volume-Responsive Low FENa States

• Low FENa does not always indicate volume-responsive prerenal azotemia 2
• Can occur with hepatic failure, cardiac failure, acute glomerulonephritis, pigment nephropathy, contrast nephrotoxicity, acute obstruction, and renal transplant rejection 2
• Non-oliguric acute renal failure may present with low FENa despite intrinsic injury 2

Chronic Kidney Disease

• Patients with chronic renal insufficiency may have FENa >1% despite volume depletion 2
• Baseline kidney function affects FENa interpretation—GFR and sodium intake predict over 80% of FENa variation 7

Oliguria vs Non-Oliguria

• Non-oliguric patients with acute tubulointerstitial nephritis have elevated FENa in less than half of cases 4
• FENa values are particularly low in non-oliguric patients with mild renal impairment 4

Integration with Other Clinical Data

• Always interpret FENa alongside volume status assessment, medication history (especially diuretics), and other laboratory markers 2
• High FENa correlates with severe morphological tubular changes on renal biopsy 4
• Cannot distinguish acute tubulointerstitial nephritis from acute glomerulonephritis by FENa alone, as both show variable values 4
• Consider urine creatinine for calculating creatinine clearance and standardizing urinary biomarker concentrations 1