Is a Serum Sodium of 135 mEq/L Normal?
A serum sodium level of 135 mEq/L is at the lower boundary of the normal range and technically represents the threshold for hyponatremia, though it is generally considered acceptable and typically does not require treatment in asymptomatic patients. 1
Definition and Clinical Significance
- Hyponatremia is formally defined as serum sodium <135 mEq/L, making 135 mEq/L the exact cutoff point 1, 2
- The normal reference range for serum sodium is 135-145 mEq/L 3
- A value of 135 mEq/L sits precisely at this threshold, representing borderline normal/mild hyponatremia 1
Clinical Context Matters
When 135 mEq/L is Acceptable
- In chronic conditions like cirrhosis, sodium levels of 130-135 mEq/L are often tolerated without intervention, particularly in asymptomatic patients who frequently remain stable at these levels 1
- For patients with chronic hyponatremia, this range is generally acceptable and often does not require active treatment beyond managing the underlying condition 1
- Many cirrhotic patients with chronic hyponatremia at 130-135 mEq/L remain asymptomatic and stable without specific treatment 1
When 135 mEq/L Warrants Attention
- Even mild hyponatremia (130-135 mEq/L) should not be ignored, as it increases fall risk (21% vs 5% in normonatremic patients) and is associated with neurocognitive problems including attention deficits 1
- Hyponatremia at any level is associated with increased mortality, with sodium levels <130 mEq/L linked to a 60-fold increase in fatality (11.2% vs 0.19%) 1
- In neurosurgical patients, even mild hyponatremia at 135 mEq/L requires closer monitoring as it may indicate underlying pathology such as cerebral salt wasting or SIADH 1
Management Approach at 135 mEq/L
- For patients on diuretics with sodium 126-135 mEq/L and normal creatinine, continue diuretic therapy but monitor serum electrolytes closely—water restriction is not recommended at this level 1
- Hyponatremia should be further investigated and treated when serum sodium falls below 131 mEq/L, though monitoring is appropriate at 135 mEq/L 1
- The key is to assess the patient's volume status (hypovolemic, euvolemic, or hypervolemic), symptom severity, and underlying conditions to determine if intervention is needed 2
Common Pitfalls to Avoid
- Ignoring mild hyponatremia (135 mEq/L) as clinically insignificant is a common error, particularly in high-risk populations 1
- Failing to monitor trends—a sodium of 135 mEq/L that is stable differs from one that is declining 1
- Not considering the underlying cause, as even borderline values may indicate worsening hemodynamic status in conditions like liver disease 1