Unit Conversion: Sodium 131 mmol/L to mEq/L
A sodium concentration of 131 mmol/L is numerically equivalent to 131 mEq/L—no conversion is needed because sodium is a monovalent cation.
Why the Values Are Identical
For monovalent ions like sodium (Na⁺), the millimole (mmol) and milliequivalent (mEq) are interchangeable units because each sodium ion carries a single positive charge 1. The relationship is:
- 1 mmol/L of Na⁺ = 1 mEq/L of Na⁺
This direct equivalence applies to all monovalent electrolytes (sodium, potassium, chloride) but does not apply to divalent ions like calcium or magnesium, where the valence must be factored into the conversion 1.
Clinical Context of Sodium 131 mmol/L
While the conversion is straightforward, the clinical significance of this value warrants attention:
Classification and Monitoring
- Sodium 131 mmol/L falls just below the normal range (135–145 mmol/L) and represents mild hyponatremia 2, 3.
- The American College of Neurosurgery recommends that hyponatremia should be further investigated and treated when serum sodium is less than 131 mmol/L, though even mild reductions warrant close monitoring 4.
- Even mild hyponatremia in the 130–135 mmol/L range carries clinical implications, including a 21% fall risk compared to 5% in normonatremic patients 5.
When to Act
- For sodium 131–135 mmol/L, continue current therapy but monitor serum electrolytes closely, checking sodium levels weekly initially, then adjusting frequency based on stability 5, 6.
- Diuretics should be discontinued or adjusted if serum sodium drops below 125 mmol/L 5.
- In patients with cirrhosis, mild hyponatremia may indicate worsening hemodynamic status and requires closer surveillance 5, 6.
Practical Takeaway
The numerical value remains 131 in both unit systems (mmol/L = mEq/L for sodium), but this level sits at the threshold where clinical vigilance increases, particularly in patients on diuretics, with liver disease, or following neurosurgical procedures 4, 5, 6.