Is a serum sodium level of 131 mmol/L indicative of hyponatremia?

Is Sodium 131 mmol/L Low?

Yes, a serum sodium of 131 mmol/L is definitively low and meets the diagnostic threshold for hyponatremia, warranting full evaluation and treatment. 1, 2

Definition and Clinical Significance

Hyponatremia is defined as serum sodium <135 mmol/L, making 131 mmol/L clearly abnormal. 3, 4, 5 This level falls into the moderate hyponatremia category (125-129 mmol/L by some classifications, though 131 mmol/L sits just above this threshold). 3

A sodium level of 131 mmol/L specifically merits full evaluation and treatment according to neurosurgical guidelines. 1, 2 This is not a borderline value that can be dismissed—even mild hyponatremia (130-135 mmol/L) is associated with:

• Increased mortality risk: 60-fold increase in hospital fatality (11.2% vs 0.19% in normonatremic patients) 2
• Increased fall risk: 21% in hyponatremic patients vs 5% in normonatremic patients 2, 5
• Cognitive impairment and gait disturbances 5
• Higher fracture rates over long-term follow-up 5

Required Diagnostic Workup

At sodium 131 mmol/L, you must obtain: 1, 2

• Serum and urine osmolality to exclude pseudohyponatremia 2
• Urine sodium concentration to guide treatment decisions 2
• Urine electrolytes for diagnostic classification 2
• Serum uric acid (levels <4 mg/dL suggest SIADH with 73-100% positive predictive value) 2
• Assessment of extracellular fluid volume status through physical examination (orthostatic hypotension, dry mucous membranes, skin turgor, jugular venous distention, peripheral edema, ascites) 2

Do NOT obtain ADH or natriuretic peptide levels—these are not supported by evidence and should not delay treatment. 2

Treatment Approach Based on Volume Status

Hypovolemic Hyponatremia

• Administer isotonic saline (0.9% NaCl) for volume repletion 2, 3
• Initial rate: 15-20 mL/kg/h, then 4-14 mL/kg/h based on response 2
• Urine sodium <30 mmol/L predicts good response (71-100% positive predictive value) 2

Euvolemic Hyponatremia (SIADH)

• Fluid restriction to 1 L/day is the cornerstone of treatment 1, 2
• If no response, add oral sodium chloride 100 mEq three times daily 2
• Consider pharmacological options: urea, demeclocycline, lithium, or loop diuretics for resistant cases 1, 2

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)

• Fluid restriction to 1-1.5 L/day for sodium <125 mmol/L 2
• Discontinue diuretics temporarily if sodium <125 mmol/L 2
• Consider albumin infusion in cirrhotic patients 2
• Avoid hypertonic saline unless life-threatening symptoms present 2

Critical Correction Rate Guidelines

The single most important safety principle: NEVER exceed 8 mmol/L correction in 24 hours. 1, 2, 3, 5

• Target correction rate: 4-6 mmol/L per day for most patients 2
• High-risk patients (advanced liver disease, alcoholism, malnutrition, prior encephalopathy): 4-6 mmol/L per day maximum 2
• Exceeding these limits risks osmotic demyelination syndrome, a devastating neurological complication 1, 2, 5

Monitoring Protocol

• Check sodium levels every 24 hours initially for asymptomatic or mildly symptomatic patients 2
• Every 4-6 hours if symptoms are present 2
• Every 2 hours if severe symptoms develop 2

Special Populations Requiring Extra Caution

Neurosurgical Patients

• Distinguish between SIADH and cerebral salt wasting (CSW) as they require opposite treatments 1, 2
• CSW requires volume and sodium replacement, NOT fluid restriction 1, 2
• In subarachnoid hemorrhage patients at risk for vasospasm, NEVER use fluid restriction 1, 2
• Consider fludrocortisone or hydrocortisone in these patients 1, 2

Cirrhotic Patients

• Sodium ≤130 mmol/L increases risk of spontaneous bacterial peritonitis (OR 3.40), hepatorenal syndrome (OR 3.45), and hepatic encephalopathy (OR 2.36) 2
• Require even more cautious correction (4-6 mmol/L per day) 2

Common Pitfalls to Avoid

• Ignoring mild hyponatremia (131 mmol/L) as clinically insignificant 2
• Using fluid restriction in cerebral salt wasting—this worsens outcomes 1, 2
• Inadequate monitoring during active correction 2
• Failing to recognize and treat the underlying cause 2
• Overly rapid correction exceeding 8 mmol/L in 24 hours 1, 2