A 16-year-old in the ED has a potassium level of 2.9 mEq/L. Which laboratory result is most concerning and should be reported immediately?

• A. Potassium (K): 2.9 mEq/L
• B. Sodium: 142 mEq/L
• C. Glucose: 90 mg/dL
• D. Calcium: 9.5 mg/dL

Answer: (A)

Potassium imbalance directly affects heart function, so it’s the most urgent finding here. A potassium level of 2.9 mEq/L is significant hypokalemia and increases the risk of dangerous cardiac arrhythmias; in the ED, that warrants immediate attention with ECG monitoring and prompt, careful potassium replacement after assessing kidney function and fluid status. The other values—sodium 142, glucose 90, calcium 9.5—are within normal ranges and do not carry the same immediate life-threatening implications, so they do not require urgent reporting.