How to evaluate a patient with hyponatremia (low serum sodium level)?

Evaluation of Hyponatremia

Begin by confirming true hyponatremia with serum osmolality measurement, then systematically assess volume status and obtain targeted urine studies to determine the underlying cause and guide treatment.

Initial Laboratory Assessment

Measure serum osmolality first to exclude pseudohyponatremia from hyperglycemia (add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL) or laboratory error 1, 2. Normal serum osmolality is 275-290 mOsm/kg 1, 2.

For confirmed hypotonic hyponatremia (serum osmolality <280 mOsm/kg), obtain:

• Urine osmolality and urine sodium concentration 1, 3, 2
• Serum creatinine, BUN, glucose 1, 2
• Thyroid-stimulating hormone (TSH) to exclude hypothyroidism 1, 2
• Serum uric acid (levels <4 mg/dL have 73-100% positive predictive value for SIADH) 1, 3

Do not routinely order plasma ADH or natriuretic peptide levels—these are not supported by evidence and delay diagnosis 1, 3.

Volume Status Assessment

Physical examination alone is unreliable (sensitivity 41.1%, specificity 80%) for determining volume status 1, 3. Look for specific clinical findings:

Hypovolemic Signs

• Orthostatic hypotension, dry mucous membranes, decreased skin turgor 1, 3
• Flat neck veins, tachycardia 1
• Urine sodium <30 mmol/L predicts 71-100% response to saline infusion 1, 3

Euvolemic Signs

• No edema, no orthostatic hypotension, normal skin turgor 1
• Moist mucous membranes 1

Hypervolemic Signs

• Peripheral edema, ascites, jugular venous distention 1, 3
• Pulmonary congestion 1

Urine Studies Interpretation

Urine Osmolality

• <100 mOsm/kg: Appropriate ADH suppression, suggests primary polydipsia or beer potomania 1, 2
• >100 mOsm/kg: Impaired water excretion, proceed to urine sodium 1, 2

Urine Sodium Concentration

For hypovolemic hyponatremia:

• <30 mmol/L: Extrarenal losses (GI losses, burns, dehydration) 1, 3, 2
• >20 mmol/L: Renal losses (diuretics, cerebral salt wasting, adrenal insufficiency) 1, 3

For euvolemic hyponatremia:

• >20-40 mmol/L with urine osmolality >300 mOsm/kg: SIADH 1, 3, 2

For hypervolemic hyponatremia:

• Urine sodium >20 mmol/L with volume overload suggests heart failure, cirrhosis, or advanced renal failure 3

Diagnostic Algorithm by Volume Status

Hypovolemic Hyponatremia

Check urine sodium 3:

• If <30 mmol/L: GI losses, burns, third-spacing 1, 2
• If >20 mmol/L: Diuretics, cerebral salt wasting, adrenal insufficiency, salt-losing nephropathy 1, 3

Euvolemic Hyponatremia

Rule out hypothyroidism (TSH) and adrenal insufficiency (cortisol) 1, 2. If excluded and urine osmolality >100 mOsm/kg with urine sodium >40 mEq/L, diagnose SIADH 1, 3.

SIADH diagnostic criteria 3:

• Serum sodium <134 mEq/L
• Plasma osmolality <275 mOsm/kg
• Urine osmolality >500 mOsm/kg (inappropriately elevated)
• Urine sodium >20-40 mEq/L
• Normal thyroid, adrenal, and renal function
• Clinical euvolemia

Hypervolemic Hyponatremia

Assess for heart failure (BNP), cirrhosis (liver function tests), or renal failure (creatinine) 1. These patients have total body sodium excess despite low serum sodium 1.

Special Considerations in Neurosurgical Patients

Distinguish SIADH from cerebral salt wasting (CSW)—they require opposite treatments 1, 3:

SIADH characteristics:

• Euvolemic state, normal CVP (6-10 cm H₂O) 1
• Urine sodium >20-40 mEq/L, urine osmolality >500 mOsm/kg 3
• Treatment: Fluid restriction 1, 3

CSW characteristics:

• Hypovolemic state, CVP <6 cm H₂O 1
• Urine sodium >20 mmol/L despite volume depletion 1, 3
• Evidence of extracellular volume depletion 1
• Treatment: Volume and sodium replacement, never fluid restriction 1, 3

Common Diagnostic Pitfalls

• Relying solely on physical examination for volume status (poor accuracy) 1, 3
• Ordering ADH levels routinely (not evidence-based, delays treatment) 1, 3
• Misdiagnosing CSW as SIADH in neurosurgical patients (leads to harmful fluid restriction) 1, 3
• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant—even mild hyponatremia increases fall risk (21% vs 5%) and mortality (60-fold increase with sodium <130 mmol/L) 1, 4
• Failing to check serum osmolality first to exclude pseudohyponatremia 1, 2

When to Pursue Full Workup

Investigate and treat when serum sodium drops below 131 mmol/L 1, 3, though even mild hyponatremia (130-135 mmol/L) warrants attention due to increased morbidity 1, 4.