Hyponatremia is defined as serum sodium below which value?

• A. 135 mEq/L
• B. 145 mEq/L
• C. 125 mEq/L
• D. 155 mEq/L

Answer: (A)

Hyponatremia occurs when the sodium level in the blood is too low, reflecting an excess of water relative to sodium in the body. The standard lab threshold used to define hyponatremia is a serum sodium concentration below 135 mEq/L. Normal sodium typically ranges from about 135 to 145 mEq/L, so dropping below 135 means hyponatremia.

In this context, the cutoff value is 135. The other numbers represent either normal range (145), a higher-than-normal value (155, which is hypernatremia), or a lower sodium value that would be hyponatremia but not the definitional threshold itself (125).