What blood tests are used to diagnose and manage hyponatremia (low sodium levels)?

Essential Blood Tests for Diagnosing and Managing Hyponatremia

A comprehensive diagnostic workup for hyponatremia should include serum and urine osmolality, urine electrolytes, and serum uric acid level as the core laboratory tests to determine the cause and guide appropriate management. 1

Initial Diagnostic Tests

Core Blood Tests

• Serum sodium level (confirms hyponatremia when <135 mmol/L)
• Serum osmolality (to differentiate true from pseudohyponatremia)
• Serum uric acid (levels <4 mg/dL have high positive predictive value for SIADH) 2
• Serum creatinine (to assess renal function and guide management)
• Blood urea nitrogen (BUN) (helps assess volume status)

Additional Blood Tests

• Complete blood count (to evaluate for infection or other causes)
• Liver function tests (to assess for liver disease as a cause of hypervolemic hyponatremia)
• Thyroid function tests (TSH, free T4) (hypothyroidism can cause hyponatremia)
• Morning cortisol level (to rule out adrenal insufficiency)
• Glucose (hyperglycemia can cause pseudohyponatremia)
• Lipid profile (hyperlipidemia can cause pseudohyponatremia)
• Serum protein electrophoresis (if multiple myeloma is suspected)

Urine Studies

• Urine osmolality (key for differentiating causes)
• Urine sodium concentration (critical for determining volume status)
• Urine potassium
• Fractional excretion of sodium and urea

Volume Status Classification

Determining volume status is crucial for diagnosis and management:

Volume Status	Urine Osmolality	Urine Sodium	Clinical Signs	Likely Diagnosis
Hypovolemic	Variable	<20 mEq/L	Orthostatic hypotension, dry mucous membranes	Volume depletion
Euvolemic	>500 mOsm/kg	>20-40 mEq/L	No edema, normal vital signs	SIADH
Hypervolemic	Elevated	<20 mEq/L	Edema, ascites, elevated JVP	Heart failure, cirrhosis

Diagnostic Algorithm

1. Confirm true hyponatremia:

   • Check serum osmolality
   • If normal/high (>280 mOsm/kg): Consider pseudohyponatremia (hyperlipidemia, hyperproteinemia) or translocational hyponatremia (hyperglycemia, mannitol)
   • If low (<280 mOsm/kg): Proceed with hypotonic hyponatremia workup 3

2. Assess volume status:

   • Physical examination findings alone are insufficient (sensitivity only 41.1%, specificity 80%) 2
   • Use combination of clinical and laboratory parameters

3. Evaluate urine studies:

   • Urine osmolality <100 mOsm/kg suggests appropriate ADH suppression (primary polydipsia)
   • Urine osmolality >100 mOsm/kg suggests inappropriate ADH secretion 3
   • Urine sodium <30 mmol/L has 71-100% positive predictive value for response to saline infusion 2

4. Specific diagnostic considerations:

   • For neurosurgical patients: Consider cerebral salt wasting (CSW) vs. SIADH
   • For cirrhotic patients: Monitor sodium levels closely when on diuretics 2

Special Considerations

Neurosurgical Patients

For neurosurgical patients, central venous pressure (CVP) measurement may help distinguish between SIADH (CVP 6-10 cm H₂O) and cerebral salt wasting (CVP <6 cm H₂O) 2. However, determination of extracellular fluid status using physical examination alone has been shown to be inaccurate 2.

Cirrhotic Patients

In cirrhotic patients with ascites, serum sodium monitoring is critical, especially when on diuretic therapy:

• Serum sodium 126-135 mmol/L: Continue diuretics but monitor electrolytes
• Serum sodium 121-125 mmol/L: Consider stopping diuretics
• Serum sodium <120 mmol/L: Stop diuretics and consider volume expansion 2

Monitoring During Treatment

• Check serum sodium levels every 2-4 hours initially during active correction 1
• Avoid increasing serum sodium by >8 mEq/L in 24 hours (or >12 mmol/L per 24 hours in cirrhotic patients) 2, 1
• For high-risk patients (alcoholics, malnourished, liver disease), limit correction to 4-6 mEq/L per day 1
• Continue monitoring electrolytes after initial correction to prevent recurrence

Pitfalls to Avoid

1. Misdiagnosis of volume status: Physical examination alone is insufficient; combine with laboratory parameters 2
2. Failure to identify pseudohyponatremia: Always check serum osmolality 4
3. Measuring ADH levels: Not recommended as it has limited diagnostic value 2
4. Overly rapid correction: Can lead to osmotic demyelination syndrome with serious neurological consequences 1
5. Inadequate monitoring: Especially during active correction of severe hyponatremia 1

By following this systematic approach to laboratory diagnosis and monitoring, clinicians can accurately identify the cause of hyponatremia and implement appropriate management strategies to improve patient outcomes.