Pseudohyponatremia in the Setting of Hypercholesterolemia

ABSTRACT 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 (PNa) 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 PNa level of 132 mmol/L. PNa concentration can be measured using direct or indirect ISE. With indirect ISE, the sample is diluted before analysis and the PNa 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 PNa levels. Direct ISE analyzes the sample without dilution. Thus, the PNa 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; PNa = plasma sodium; SIADH = syndrome of inappropriate antidiuretic hormone secretion