What is the initial step in the workup of an adult patient presenting with hyponatremia?

Hyponatremia Workup

Initial Diagnostic Step

The first step in evaluating hyponatremia is to assess the patient's volume status through physical examination and obtain serum osmolality, urine osmolality, and urine sodium concentration. 1

Essential Initial Laboratory Tests

The initial workup should include the following tests 1:

• Serum osmolality - to exclude pseudohyponatremia and determine if hyponatremia is hypotonic, isotonic, or hypertonic 2, 3
• Urine osmolality - values <100 mOsm/kg indicate appropriate ADH suppression, while >100 mOsm/kg suggests impaired water excretion 1
• Urine sodium concentration - helps differentiate between causes of hyponatremia 1, 2
• Serum uric acid - levels <4 mg/dL have a 73-100% positive predictive value for SIADH 1
• Assessment of extracellular fluid (ECF) volume status - critical for classification 1

Volume Status Assessment

Physical examination should specifically evaluate for 1:

• Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins
• Hypervolemic signs: peripheral edema, ascites, jugular venous distention, pulmonary congestion
• Euvolemic state: absence of both hypovolemic and hypervolemic signs

Note that physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%) for volume assessment and should be supplemented with laboratory findings 1.

Interpretation of Urine Studies

Urine sodium interpretation 1, 2:

• <30 mmol/L in hypovolemic patients - suggests extrarenal losses (GI losses, burns, dehydration) and predicts 71-100% response to saline infusion
• >20 mmol/L in hypovolemic patients - suggests renal losses (diuretics, salt-wasting nephropathy)
• >20-40 mmol/L in euvolemic patients with high urine osmolality (>300 mOsm/kg) - supports SIADH diagnosis

Urine osmolality interpretation 1:

• <100 mOsm/kg - appropriate ADH suppression (primary polydipsia, reset osmostat)
• >100 mOsm/kg - impaired water excretion due to elevated ADH or decreased solute intake

Classification Algorithm

Once initial tests are obtained, classify hyponatremia as 1, 2:

1. Hypertonic hyponatremia (high serum osmolality) - caused by hyperglycemia; add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL 1

2. Isotonic hyponatremia (normal serum osmolality) - pseudohyponatremia from hyperlipidemia or hyperproteinemia 2

3. Hypotonic hyponatremia (low serum osmolality) - then further classify by volume status:

   • Hypovolemic: urine sodium <30 mmol/L (extrarenal losses) or >20 mmol/L (renal losses) 1
   • Euvolemic: urine sodium >20-40 mmol/L with urine osmolality >300 mOsm/kg suggests SIADH 1
   • Hypervolemic: heart failure, cirrhosis, nephrotic syndrome 1

Critical Threshold for Full Workup

Hyponatremia should be fully investigated and treated when serum sodium is <131 mmol/L, though even mild hyponatremia (130-135 mmol/L) requires attention as it increases fall risk (21% vs 5%) and mortality (60-fold increase with sodium <130 mmol/L) 1, 4.

Special Considerations in Neurosurgical Patients

In patients with CNS pathology, distinguish SIADH from cerebral salt wasting (CSW) as they require opposite treatments 1:

• SIADH: euvolemic, CVP normal to slightly elevated, urine sodium >20-40 mmol/L, treat with fluid restriction
• CSW: hypovolemic, CVP <6 cm H₂O, urine sodium >20 mmol/L despite volume depletion, treat with volume and sodium replacement

Common Pitfalls to Avoid

• Failing to assess volume status accurately - this is essential for determining appropriate treatment 1
• Obtaining ADH and natriuretic peptide levels - not supported by evidence and should not delay treatment 1
• Ignoring mild hyponatremia (130-135 mmol/L) - associated with cognitive impairment, falls, and fractures 1, 4
• Relying solely on physical examination for volume assessment without laboratory confirmation 1