What is the threshold alert value for low plasma potassium?

• A. 3.5 mmol/L
• B. 1.5 mmol/L
• C. 2.0 mmol/L
• D. 2.5 mmol/L

Answer: (C)

Potassium is tightly regulated because even small decreases affect cardiac electrical activity and muscle function. The threshold used to flag a potential problem is set well below the lower limit of the normal range to prompt urgent review. Normal plasma potassium is about 3.5–5.0 mmol/L, and as levels fall, the risk of dangerous arrhythmias and weakness increases. An alert value of 2.0 mmol/L is chosen because it signals a high-risk situation that justifies immediate clinical intervention. Values around 3.5 are at the lower end of normal, so they don’t trigger an alert. Very low levels like 1.5 are typically considered critical and demand rapid response, while 2.5 may be used in some settings as a less urgent threshold, but the standard alert threshold in many contexts is 2.0 mmol/L.