Pseudohyponatremia in the Setting of Hypercholesterolemia

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Objective: To describe a case of pseudohyponatremia in the setting of hypercholesterolemia. Methods: A 69-year-old man was admitted to the hospital after asymptomatic hyponatremia was detected on follow-up blood work for acute, drug-induced cholestatic hepatitis. Results: Plasma sodium (P Na ) was 119 mmol/L and total protein was 4.7 g/dL. Plasma osmolality was 283 mOsm/kg, urine osmolality was 332 mOsm/kg, and random urine sodium was 45 mmol/L. Pseudohyponatremia was suspected and lipid profiling showed triglycerides were 281 mg/dL, cholesterol was 1,340 mg/dL, high-density lipoprotein was 21 mg/dL, and low-density lipoprotein was 1,419 mg/dL. Direct ion-selective electrode (ISE) using a blood gas analyzer revealed a P Na level of 132 mmol/L. P Na concentration can be measured using direct or indirect ISE. With indirect ISE, the sample is diluted before analysis and the P Na is measured with the assumption that plasma is composed of 93% water. When hyperlipidemia is present, the actual plasma water fraction is decreased, and therefore using indirect ISE will result in falsely low P Na levels. Direct ISE analyzes the sample without dilution. Thus, the P Na is measured directly irrespective of plasma water fraction and so is not affected by changes in plasma percentage concentration. Conclusion: Around two thirds of laboratories in the United States use indirect ISE. Awareness of pseudohyponatremia is important to prevent inappropriate management. Abbreviations: ISE = ion-selective electrode; P Na = plasma sodium; SIADH = syndrome of inappropriate antidiuretic hormone secretion

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