Laboratory Evaluation for Hyponatremia
The essential laboratory workup for hyponatremia should include serum sodium, serum osmolality, urine sodium, and urine osmolality to determine the type of hyponatremia and guide appropriate treatment. 1
Initial Laboratory Assessment
The following tests should be ordered for a patient presenting with hyponatremia:
Basic laboratory tests:
- Serum sodium
- Serum osmolality
- Urine sodium
- Urine osmolality
Additional tests to identify underlying causes:
- Complete blood count
- Comprehensive metabolic panel (including renal function)
- Thyroid function tests (TSH, free T4)
- Morning cortisol level (to rule out adrenal insufficiency)
- Liver function tests
Interpretation Based on Volume Status
Laboratory results should be interpreted in the context of the patient's volume status, which can be categorized into three types:
| Volume Status | Clinical Signs | Urine Sodium | Likely Causes |
|---|---|---|---|
| Hypovolemic | Orthostatic hypotension, dry mucous membranes, tachycardia | <20 mEq/L | GI losses, diuretics, CSW, adrenal insufficiency |
| Euvolemic | No edema, normal vital signs | >20-40 mEq/L | SIADH, hypothyroidism, adrenal insufficiency |
| Hypervolemic | Edema, ascites, elevated JVP | <20 mEq/L | Heart failure, cirrhosis, renal failure |
Diagnostic Criteria for Specific Causes
SIADH Diagnostic Criteria
When SIADH is suspected, confirm the following laboratory findings 1:
- Serum sodium <134 mEq/L
- Plasma osmolality <275 mosm/kg
- Inappropriately high urine osmolality (>500 mosm/kg)
- Inappropriately high urinary sodium (>20 mEq/L)
- Normal volume status
Cerebral Salt Wasting (CSW) Diagnostic Criteria
CSW is characterized by 1:
- Hyponatremia
- Excessive natriuresis
- Volume contraction
- Low central venous pressure (CVP <6 cm H₂O)
Common Pitfalls to Avoid
Failing to check urine studies: Urine sodium and osmolality are crucial for differentiating between the various causes of hyponatremia 1, 2
Not considering pseudohyponatremia: In cases of hyperglycemia, hyperlipidemia, or hyperproteinemia, measured sodium may be falsely low. Always check serum glucose, lipids, and protein levels when appropriate 2
Overlooking medication-induced hyponatremia: Many medications can cause hyponatremia (diuretics, antidepressants, antipsychotics, anticonvulsants). A thorough medication review is essential 2, 3
Not assessing severity: The severity of hyponatremia should be classified as 1:
- Mild: 126-135 mEq/L (often asymptomatic)
- Moderate: 120-125 mEq/L (nausea, headache, confusion)
- Severe: <120 mEq/L (risk of seizures, coma, respiratory arrest)
Laboratory Monitoring During Treatment
- For symptomatic or severe hyponatremia, serum sodium should be monitored every 2-4 hours during initial treatment 1, 4
- For asymptomatic or mild hyponatremia, daily monitoring may be sufficient 1
- The rate of sodium correction should not exceed 10 mmol/L/day to prevent osmotic demyelination syndrome 1, 2
Special Considerations
- In patients with liver disease, additional tests to assess liver function and portal hypertension may be needed 1
- In patients with heart failure, measurement of BNP/NT-proBNP can help confirm the diagnosis 2
- In patients with neurological symptoms, consider brain imaging to rule out intracranial pathology, especially when cerebral salt wasting is suspected 1
The laboratory evaluation of hyponatremia is critical for determining the underlying cause and guiding appropriate treatment. By systematically assessing volume status and interpreting laboratory values in this context, clinicians can develop targeted treatment strategies to safely correct sodium levels while addressing the underlying etiology.