What unit is used to express serum potassium levels?

• A. mg/dL
• B. mEq/L
• C. mmol/L
• D. g/dL

Answer: (B)

Potassium is an electrolyte, and its levels are reported using a unit that reflects electrical charge per liter. For potassium, the standard reporting unit is milliequivalents per liter (mEq/L) because it directly relates to the ion’s physiologic activity and helps with dosing and replacement decisions. Potassium has a valence of +1, so 1 mmol of K+ equals 1 mEq of K+, making mEq/L the appropriate way to express its concentration. Some labs may report mmol/L, which is numerically similar for potassium, but the nursing and clinical convention is mEq/L. Units like mg/dL or g/dL are mass-based and do not capture the ion’s charge or its role in body fluid balance, so they’re not used for serum potassium. Typical reference range is about 3.5–5.0 mEq/L.