What labs and imaging studies should be ordered for a patient with hyponatremia (low sodium levels)?

Laboratory and Imaging Workup for Hyponatremia

Order serum osmolality, urine osmolality, urine sodium, serum uric acid, and assess volume status clinically—imaging is generally not required for hyponatremia evaluation unless investigating an underlying cause like malignancy or CNS pathology. 1

Essential Initial Laboratory Tests

First-Line Serum Tests

• Serum osmolality to confirm true hypotonic hyponatremia and exclude pseudohyponatremia (normal range 275-290 mOsm/kg) 1, 2
• Serum sodium level to quantify severity: mild (130-135 mEq/L), moderate (125-129 mEq/L), severe (<125 mEq/L) 3
• Serum creatinine and BUN to assess renal function and identify hypovolemia (often elevated in hypovolemic states) 1
• Serum glucose to exclude hyperglycemia-induced hyponatremia 1
• Serum uric acid: levels <4 mg/dL have 73-100% positive predictive value for SIADH (though may also occur in cerebral salt wasting) 1, 4
• Thyroid-stimulating hormone (TSH) to rule out hypothyroidism 1
• Morning cortisol level to exclude adrenal insufficiency 1, 2

Critical Urine Tests

• Urine osmolality: <100 mOsm/kg indicates appropriate ADH suppression; >300-500 mOsm/kg suggests SIADH or volume depletion 1, 4, 2
• Urine sodium concentration:
  • <20-30 mmol/L suggests hypovolemic hyponatremia with 71-100% positive predictive value for response to normal saline 1, 4

  • 20-40 mmol/L suggests SIADH, cerebral salt wasting, diuretic use, or adrenal insufficiency 1, 4, 2

• 24-hour urine sodium collection (or spot urine sodium/potassium ratio >1) can confirm total sodium excretion, particularly useful in cirrhotic patients 1

Additional Laboratory Tests Based on Clinical Context

• Liver function tests (including albumin) if cirrhosis suspected 1
• Brain natriuretic peptide (BNP) if heart failure suspected 1
• Complete blood count for baseline assessment 1
• Serum potassium, calcium, magnesium to identify concurrent electrolyte abnormalities 1
• Lipid profile in select cases 1

Volume Status Assessment (Clinical, Not Imaging)

Physical examination is essential but has limited accuracy (sensitivity 41%, specificity 80%) for determining volume status. 1, 4 Look for:

Hypovolemic Signs

• Orthostatic hypotension (postural pulse change or severe dizziness preventing standing) 1
• Dry mucous membranes and furrowed tongue 1
• Decreased skin turgor 1
• Sunken eyes 1
• Low blood pressure and decreased venous filling 1

Hypervolemic Signs

• Jugular venous distention 1
• Peripheral edema 1
• Ascites 1
• Orthopnea and dyspnea 1

Euvolemic Appearance

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

Imaging Studies

Imaging is NOT routinely required for hyponatremia workup itself. 1 However, consider imaging in specific clinical scenarios:

When to Order Imaging

• Chest X-ray or CT chest: If lung cancer suspected as cause of SIADH (affects 1-5% of lung cancer patients, particularly small cell) 1
• Brain MRI:
  • If cerebral salt wasting or SIADH from CNS pathology suspected (subarachnoid hemorrhage, brain tumor, meningitis) 1
  • If osmotic demyelination syndrome suspected (look for dysarthria, dysphagia, oculomotor dysfunction, quadriparesis typically 2-7 days after rapid correction) 1
• Abdominal ultrasound or CT: If cirrhosis with ascites suspected 1
• Central venous pressure (CVP) measurement: In neurosurgical patients to distinguish SIADH (CVP 6-10 cm H₂O) from cerebral salt wasting (CVP <6 cm H₂O) 4

Tests NOT Recommended

Do NOT routinely order ADH levels or natriuretic peptide levels—these are not supported by evidence and do not change management. 1, 4

Diagnostic Algorithm Based on Lab Results

Step 1: Confirm True Hyponatremia

• If serum osmolality normal or high (≥280 mOsm/kg): consider pseudohyponatremia from hyperglycemia, hypertriglyceridemia, or lab error 1, 2
• If serum osmolality low (<280 mOsm/kg): proceed to Step 2 2

Step 2: Check Urine Osmolality

• <100 mOsm/kg: Primary polydipsia or reset osmostat 1, 2
• >300 mOsm/kg: Impaired water excretion—proceed to Step 3 1, 2

Step 3: Determine Volume Status + Urine Sodium

• Hypovolemic + urine Na <30 mmol/L: Extrarenal losses (vomiting, diarrhea, third-spacing) 4
• Hypovolemic + urine Na >20 mmol/L: Renal losses (diuretics, cerebral salt wasting, adrenal insufficiency, salt-losing nephropathy) 4
• Euvolemic + urine Na >40 mmol/L + urine osm >500 mOsm/kg: SIADH (also check TSH, cortisol to exclude hypothyroidism/adrenal insufficiency) 4, 2
• Hypervolemic + urine Na >20 mmol/L: Advanced heart failure, cirrhosis, or renal failure 4

Monitoring During Treatment

• Severe symptoms: Check serum sodium every 2 hours during initial correction 1
• After symptom resolution: Check every 4 hours 1
• Chronic/asymptomatic: Check daily to ensure correction does not exceed 8 mmol/L in 24 hours 1, 5
• High-risk patients (cirrhosis, alcoholism, malnutrition): Aim for 4-6 mmol/L per day maximum 1

Common Pitfalls to Avoid

• Relying solely on physical exam for volume status determination—combine with urine sodium and clinical context 1, 4
• Ordering ADH or natriuretic peptide levels—these do not guide management 1
• Ignoring mild hyponatremia (130-135 mmol/L)—even mild levels increase fall risk (21% vs 5%) and mortality 1, 6
• Failing to check urine studies—serum tests alone cannot distinguish SIADH from cerebral salt wasting or determine volume status 1, 4
• Not monitoring frequently enough during correction—inadequate monitoring is a major cause of osmotic demyelination syndrome 1