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Classification: Low (hyponatremia)
Sodium 128 is hyponatremia needing evaluation — the cause (fluid overload, diuretics, or water excess) determines whether to restrict fluid or adjust medications.
Sodium reflects water balance — the ratio of total body sodium to total body water. Kidneys regulate both; kidney disease disturbs the balance.
Standard blood test.
Reference: Normal range per major laboratories; correction guidance per Adrogué-Madias • Normal range: 135-145 mEq/L