What laboratory tests are indicated for patients with hyponatremia (low sodium levels)?

Laboratory Evaluation for Hyponatremia

All patients with hyponatremia require a systematic laboratory workup that includes serum osmolality, urine osmolality, urine sodium, and assessment of renal function and electrolytes to determine the underlying cause and guide treatment. 1

Initial Essential Laboratory Tests

Serum Studies:

• Serum osmolality - First critical step to differentiate hypotonic from non-hypotonic hyponatremia (threshold: <280 mOsm/kg indicates true hypotonic hyponatremia) 2, 3
• Serum sodium, potassium, chloride - Baseline electrolyte assessment 4, 1
• Blood urea nitrogen (BUN) and creatinine - Evaluate renal function and volume status 4
• Glucose - Rule out hyperglycemia causing pseudohyponatremia 4, 2
• Lipid profile - Exclude pseudohyponatremia from severe hyperlipidemia 2

Urine Studies:

• Urine osmolality - Determines if water excretion is appropriate (values <100 mOsm/kg indicate appropriate ADH suppression; >100 mOsm/kg suggests impaired water excretion) 2, 3
• Urine sodium concentration - Critical for determining volume status and etiology (spot urine sodium <30 mmol/L suggests hypovolemia with 71-100% positive predictive value for saline responsiveness; >40 mmol/L suggests SIADH or renal salt wasting) 1, 2

Additional Diagnostic Tests Based on Clinical Context

Hormonal Assessment:

• Thyroid-stimulating hormone (TSH) - Rule out hypothyroidism as a cause of euvolemic hyponatremia 4, 1
• Morning cortisol level - Exclude adrenal insufficiency in difficult cases 2, 3

Cardiac and Hepatic Evaluation:

• Liver function tests (AST, ALT, bilirubin, albumin) - Assess for cirrhosis in hypervolemic hyponatremia 4, 1
• Brain natriuretic peptide (BNP) - Evaluate for heart failure when hypervolemic hyponatremia is suspected 1

Additional Markers:

• Serum uric acid - Values <4 mg/dL have 73-100% positive predictive value for SIADH (though may also occur in cerebral salt wasting) 1
• Complete blood count - Assess for anemia or infection 4, 1

Advanced Diagnostic Parameters

For Difficult Cases:

• Fractional excretion of urea - Helps differentiate SIADH from volume depletion 3
• Fractional uric acid excretion - May improve diagnostic accuracy in distinguishing SIADH from other causes 3
• 24-hour urine sodium collection - Determines total sodium excretion (can be replaced by spot urine sodium/potassium ratio >1, which correlates with 24-hour sodium excretion >78 mmol/day with ~90% accuracy) 1

Volume Status Assessment

Physical examination findings guide laboratory interpretation:

• Hypovolemia indicators: Orthostatic hypotension, dry mucous membranes, decreased skin turgor, tachycardia 1, 2
• Euvolemia indicators: No edema, normal blood pressure, moist mucous membranes 1, 2
• Hypervolemia indicators: Jugular venous distention, peripheral edema, ascites, pulmonary congestion 1, 2

Monitoring During Treatment

Serial sodium measurements are essential:

• Every 2 hours during initial correction for severe symptomatic hyponatremia 1
• Every 4 hours after resolution of severe symptoms 1
• Every 1-2 days for hospitalized patients with chronic hyponatremia 4

Common Diagnostic Pitfalls to Avoid

• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant, when it actually increases fall risk and mortality 1, 5
• Failing to measure urine studies before initiating treatment, which can obscure the diagnosis 2, 3
• Not checking serum osmolality first to exclude pseudohyponatremia from hyperglycemia or hyperlipidemia 2, 3
• Obtaining ADH or natriuretic peptide levels routinely, as these are not supported by evidence and should not delay treatment 1
• Misinterpreting urine sodium in patients on diuretics, which can be elevated despite true volume depletion 2, 6