How to manage a patient with a Fractional Excretion of Sodium (Fena) of 0.20?

Management of FENa 0.20%

A FENa of 0.20% indicates avid sodium retention by the kidneys, suggesting either prerenal azotemia (volume depletion) or a sodium-retaining state such as heart failure, cirrhosis, or nephrotic syndrome—immediate assessment of volume status and clinical context is essential to determine whether volume resuscitation or enhanced diuretic therapy is needed. 1, 2

Interpretation of FENa 0.20%

• FENa <1% classically indicates prerenal causes of acute kidney injury, including hypovolemia, with the kidneys avidly retaining sodium in response to perceived volume depletion 1, 2
• Your patient's FENa of 0.20% is well below the 1% threshold, strongly suggesting either true volume depletion or a pathologic sodium-retaining state 2
• In the original validation studies, prerenal azotemia consistently showed FENa <1% while acute tubular necrosis showed FENa >3%, making this a powerful discriminator 2

Clinical Context Determines Management

If Volume Depleted (True Prerenal State):

• Administer intravenous fluid resuscitation to restore intravascular volume and renal perfusion 3
• A marked decrease in FENa during acute kidney injury should prompt evaluation for superimposed volume depletion, even in patients with underlying intrinsic renal disease 3
• Monitor for prompt increase in urine output and improvement in renal function after fluid administration, which confirms prerenal etiology 3

If Sodium-Retaining State (Heart Failure, Cirrhosis):

• FENa <0.5% in heart failure patients indicates inadequate diuretic response and predicts need for enhanced decongestive strategies 1
• In acute decompensated heart failure, FENa >0.4% measured >6 hours after diuretic dosing predicts worsening renal impairment (OR 6.3), while FENa <0.4% suggests insufficient natriuresis 4
• Your patient's FENa of 0.20% indicates severe sodium avidity requiring intensified diuretic therapy, not volume resuscitation 5, 1

Enhanced Diuretic Strategies for Low FENa

When FENa remains low despite diuretic therapy (indicating diuretic resistance):

• Sequential nephron blockade is the recommended approach, combining loop diuretics with thiazide-type diuretics or acetazolamide to overcome adaptive sodium reabsorption mechanisms 5
• Acetazolamide acts proximally to reduce sodium reabsorption in the proximal tubule, while thiazides block distal tubular hypertrophy-mediated sodium reabsorption 5
• The ADVOR trial demonstrated that acetazolamide added to loop diuretics enhances decongestion in acute heart failure with diuretic resistance 5
• Intravenous loop diuretics at higher doses may be needed, as oral bioavailability decreases with gut edema 5

Special Populations

Cystic Fibrosis:

• Target FENa range is 0.5-1.5% to assess sodium supplementation needs 5, 1
• Your patient's FENa of 0.20% is below target, indicating need for sodium supplementation with 1-2 mmol/kg/day 5

Cirrhosis with Ascites:

• FENa <1% suggests hepatorenal syndrome with high sensitivity (100%) but poor specificity (14%) 1
• Consider fractional excretion of urea (FEUrea) as FEUrea <28% has better specificity (83%) for hepatorenal syndrome 1

Critical Pitfalls to Avoid

• Do not assume FENa <1% always means volume depletion—sodium-retaining states (heart failure, cirrhosis) also produce low FENa 1, 4
• Diuretic use invalidates standard FENa interpretation by artificially raising FENa; consider FEUrea (<35% suggests prerenal) if diuretics were recently administered 6
• Contrast-induced acute kidney injury can present with persistently low FENa (<1%) for up to 5 days despite ongoing renal dysfunction, mimicking prerenal azotemia 7
• In heart failure with advanced CKD, low FENa may reflect both reduced filtered sodium load and neurohormonal activation, requiring aggressive combination diuretic therapy rather than fluid administration 5

Practical Algorithm

1. Assess volume status clinically: orthostatic vitals, jugular venous pressure, peripheral edema, lung examination 5
2. If hypovolemic: give IV fluids and reassess renal function within 24 hours 3
3. If euvolemic/hypervolemic with heart failure or cirrhosis: intensify diuretics with sequential nephron blockade 5
4. If on diuretics: measure FEUrea (target >35% suggests ATN, <35% suggests prerenal or inadequate diuresis) 6
5. Monitor response: repeat FENa after intervention to confirm appropriate trajectory 1