How do you interpret hyponatremia labs?

Interpreting Hyponatremia Labs: A Systematic Approach

Begin with serum sodium <135 mmol/L as your threshold for hyponatremia, but pursue full workup when sodium drops below 131 mmol/L. 1, 2

Initial Laboratory Assessment

When hyponatremia is identified, obtain the following tests immediately:

• Serum osmolality to exclude pseudohyponatremia (normal: 275-290 mOsm/kg) 1, 2
• Urine osmolality to assess water excretion capacity 1, 3
• Urine sodium concentration to differentiate causes 1, 4
• Serum uric acid (values <4 mg/dL suggest SIADH with 73-100% positive predictive value) 1, 4
• Thyroid-stimulating hormone and cortisol to exclude endocrine causes 2, 3

Do not obtain ADH or natriuretic peptide levels—these have no diagnostic value and are not supported by evidence. 1, 2

Step 1: Assess Serum Osmolality

Normal or High Serum Osmolality (≥280 mOsm/kg)

This indicates pseudohyponatremia from:

• Laboratory error 1
• Hyperglycemia (add 1.6 mEq/L to sodium for each 100 mg/dL glucose >100 mg/dL) 2, 5
• Hypertriglyceridemia 1

Low Serum Osmolality (<280 mOsm/kg)

Proceed to Step 2 for true hypotonic hyponatremia 1, 3

Step 2: Determine Volume Status

Physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%) for volume assessment. 1, 4 Look for these specific findings:

Hypovolemic Signs

• Orthostatic hypotension (≥10% increase in pulse or ≥10% decrease in systolic BP from supine to standing) 1
• Dry mucous membranes 1, 4
• Decreased skin turgor 1, 4
• Flat neck veins 1

Hypervolemic Signs

• Peripheral edema 1, 2
• Ascites 1, 2
• Jugular venous distention 1, 2
• Pulmonary congestion 2

Euvolemic Appearance

• No edema 6
• Normal skin turgor 6
• Moist mucous membranes 6
• No orthostatic changes 6

Step 3: Interpret Urine Studies

Urine Osmolality

• <100 mOsm/kg: Appropriate ADH suppression; consider primary polydipsia 1, 3
• >100 mOsm/kg: Impaired water excretion; proceed to urine sodium 1, 3

Urine Sodium Concentration

For Hypovolemic Hyponatremia:

• <30 mmol/L: Extrarenal losses (GI losses, burns, dehydration); predicts 71-100% response to normal saline 1, 2, 4
• >20 mmol/L: Renal losses (diuretics, cerebral salt wasting, adrenal insufficiency, salt-losing nephropathy) 1, 4

For Euvolemic Hyponatremia:

• >20-40 mmol/L with urine osmolality >300 mOsm/kg: SIADH 4, 6
• Serum uric acid <4 mg/dL further supports SIADH (73-100% PPV) 1, 4

For Hypervolemic Hyponatremia:

• <30 mmol/L: Heart failure, cirrhosis (avid sodium retention) 2
• >20 mmol/L: Advanced renal failure 4

Step 4: Critical Distinction in Neurosurgical Patients

In patients with CNS pathology (especially subarachnoid hemorrhage), distinguish SIADH from cerebral salt wasting (CSW)—this is crucial because treatments are opposite. 1, 2, 4

SIADH (Euvolemic)

• Normal to slightly elevated central venous pressure (6-10 cm H₂O) 4, 6
• Urine sodium >20-40 mmol/L 4, 6
• Urine osmolality >500 mOsm/kg 6
• Treatment: Fluid restriction 1, 6

Cerebral Salt Wasting (Hypovolemic)

• Low central venous pressure (<6 cm H₂O) 4
• Urine sodium >20 mmol/L despite volume depletion 1, 4
• Clinical signs of hypovolemia 1, 4
• Treatment: Volume and sodium replacement (NOT fluid restriction) 1, 2, 4

Common Diagnostic Pitfalls

Avoid these errors:

• Relying solely on physical examination for volume status (sensitivity only 41%) 1, 4
• Ordering ADH levels (no diagnostic utility) 1, 2
• Missing pseudohyponatremia from hyperglycemia 2, 5
• Confusing SIADH with CSW in neurosurgical patients 1, 4
• Ignoring mild hyponatremia (130-135 mmol/L)—even this increases fall risk and mortality 2, 7

Severity Classification

• Mild: 130-135 mEq/L 2, 5
• Moderate: 125-129 mEq/L 2, 5
• Severe: <125 mEq/L 2, 5

Even mild hyponatremia (130-135 mEq/L) is clinically significant—it increases falls by 45% (23.8% vs 16.4%) and fractures by 35% (23.3% vs 17.3%) over 7 years. 7