What is the likely fractional excretion of sodium (FeNa) in a dehydrated patient?

Fractional Excretion of Sodium in Dehydrated Patients

In dehydrated patients, the fractional excretion of sodium (FeNa) is typically less than 1%, indicating the kidney's appropriate conservation of sodium and water in response to volume depletion. 1

Pathophysiology and Expected Values

Dehydration triggers compensatory mechanisms that enhance sodium and water reabsorption in the kidneys:

• When volume depletion occurs, the renin-angiotensin-aldosterone system activates
• This leads to increased sodium reabsorption in the renal tubules
• The result is decreased sodium excretion in the urine

The typical findings in a dehydrated patient include:

• FeNa < 1% (high sensitivity for volume depletion)
• Urine sodium concentration < 20 mEq/L
• Concentrated urine (high urine osmolality)

Diagnostic Accuracy and Interpretation

The diagnostic value of FeNa varies depending on clinical context:

• In oliguric patients without chronic kidney disease or diuretic therapy, FeNa has excellent diagnostic performance with:

  • Sensitivity of 95%
  • Specificity of 91% for distinguishing prerenal from intrinsic causes 2

• In patients with normal kidney function, a FeNa < 1% strongly suggests volume depletion as the cause of acute kidney injury 1

Important Clinical Caveats

Several clinical scenarios can affect the interpretation of FeNa:

1. Recent diuretic use: Can falsely elevate FeNa despite true volume depletion 3, 4

2. Chronic kidney disease: May have higher baseline FeNa values even when volume depleted 2

3. Sodium-avid states: Conditions like heart failure, cirrhosis, and nephrotic syndrome can have FeNa < 1% despite not being purely volume depleted 5, 4

4. Certain types of acute kidney injury: Some intrinsic renal injuries can present with low FeNa, including:

   • Contrast-induced nephropathy 6
   • Acute glomerulonephritis
   • Pigment nephropathy
   • Non-oliguric acute tubular necrosis 4

Alternative Diagnostic Tests

When FeNa interpretation is complicated by confounding factors:

• Fractional excretion of urea (FEUrea): Less affected by diuretic use and may better discriminate between causes of AKI

  • FEUrea < 28.16% has sensitivity of 75% and specificity of 83% in separating hepatorenal syndrome from other causes 3

• Uric acid levels: Serum uric acid < 4 mg/dL in hyponatremic states suggests SIADH rather than volume depletion 1

• Clinical assessment: Physical examination findings of dehydration include:

  • Decreased skin turgor
  • Dry mucous membranes
  • Orthostatic hypotension
  • Tachycardia 3

Clinical Algorithm for Interpretation

1. Measure FeNa in suspected dehydration
2. If FeNa < 1% and no confounding factors → confirms volume depletion
3. If FeNa < 1% with confounding factors (diuretics, CKD, heart/liver disease) → consider additional tests:
   • FEUrea
   • Clinical assessment of volume status
   • Response to fluid challenge
4. If FeNa > 1% without diuretic use → consider intrinsic renal causes or other diagnoses

Remember that while FeNa is a valuable diagnostic tool, it must be interpreted in the context of the patient's clinical presentation and other laboratory findings to guide appropriate management.