What are the lab values for hypotonic hypovolemic hyponatremia?

Lab Values for Hypotonic Hypovolemic Hyponatremia

Hypotonic hypovolemic hyponatremia is characterized by serum sodium <135 mmol/L, serum osmolality <280 mOsm/kg, urine sodium <30 mmol/L, and signs of volume depletion. 1, 2

Diagnostic Laboratory Values

Serum Parameters

• Serum sodium: <135 mmol/L (mild: 130-134 mmol/L, moderate: 125-129 mmol/L, severe: <125 mmol/L) 2
• Serum osmolality: <280 mOsm/kg (indicating hypotonic state) 1
• Blood urea nitrogen (BUN): Often elevated 1
• Serum creatinine: Often elevated due to prerenal azotemia 1
• Serum uric acid: Usually elevated (>4 mg/dL) 1

Urine Parameters

• Urine sodium: <30 mmol/L (key diagnostic feature indicating appropriate renal sodium conservation) 1, 3
• Urine osmolality: Usually >100 mOsm/kg (concentrated urine) 1
• Fractional excretion of sodium (FENa): <1% (indicating appropriate renal response to volume depletion) 1

Clinical Assessment of Volume Status

Hypotonic hypovolemic hyponatremia is characterized by the following clinical signs:

• Orthostatic hypotension 1
• Tachycardia 1
• Decreased skin turgor 1
• Dry mucous membranes 1
• Reduced jugular venous pressure 1
• Weight loss 1

Differential Diagnosis Based on Lab Values

The combination of low serum sodium, low serum osmolality, and low urine sodium (<30 mmol/L) has a positive predictive value of 71-100% for response to 0.9% saline infusion, confirming hypovolemic hyponatremia 1.

Comparison with Other Types of Hyponatremia:

1. Hypotonic Euvolemic Hyponatremia (SIADH):

   • Serum sodium: <135 mmol/L
   • Serum osmolality: <280 mOsm/kg
   • Urine sodium: >20-30 mmol/L (key differentiating factor)
   • Urine osmolality: >100 mOsm/kg, often >500 mOsm/kg
   • Serum uric acid: <4 mg/dL 1

2. Hypotonic Hypervolemic Hyponatremia:

   • Serum sodium: <135 mmol/L
   • Serum osmolality: <280 mOsm/kg
   • Urine sodium: Usually <20 mmol/L (unless on diuretics)
   • Clinical evidence of edema, ascites, or fluid overload 1, 3

Common Causes

• Gastrointestinal losses (vomiting, diarrhea) 4, 5
• Excessive diuretic use 1, 5
• Third-space losses (burns, pancreatitis) 1
• Adrenal insufficiency 1
• Cerebral salt wasting 1

Treatment Considerations

For hypovolemic hyponatremia, the American Association for the Study of Liver Diseases recommends:

• Discontinuing diuretics 1
• Administering isotonic saline (0.9% NaCl) for volume repletion 1, 3
• Limiting correction rate to <8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 6
• For severe symptoms (seizures, coma), correction by 6 mmol/L over 6 hours or until symptoms improve 1

Common Pitfalls

• Misdiagnosing volume status can lead to inappropriate treatment 1
• Using hypertonic saline in hypovolemic hyponatremia before adequate volume repletion 1
• Failing to identify and treat the underlying cause of hypovolemia 1
• Correcting sodium too rapidly, risking osmotic demyelination syndrome 1, 6