1. Presented by Cherub Kim, MD and reviewed by Dr. Claire Knezevic, PhD.

An adult patient with a history of multiple myeloma, on treatment with Daratumumab, presents with fatigue, epigastric pain, and hypoglycemia. A chemistry panel showed a serum sodium concentration of 131 mmol/L (reference range 135-148 mmol/L), potassium concentration of 4.8 mmol/L (reference range 3.5-5.1 mmol/L), chloride concentration of 106 mmol/L (reference range 96-109 mmol/L), calcium concentration of 7.1 mmol/L (reference range 8.4-10.5 mg/dL), additional labs showed total protein of 10.1 g/dL (reference range 6.0-8.2 g/dL), total cholesterol of 200 mg/dL (reference range <200 mg/dL). Whole blood testing performed simultaneously showed sodium concentration of 137 mmol/L (reference range 135-148 mmol/L), potassium concentration of 4.6 mmol/L (reference range 3.5-5.1 mmol/L), chloride of 112 mmol/L (reference range 96-109 mmol/L). What is the most commonly used method for measurement of serum sodium?