SAT-265 Lipoprotein X-induced Pseudohyponatremia In A Patient With Cholestatic Cholangiocarcinoma

Abstract Disclosure: L. Rasquin: None. Y. Wang: None. D. Carruthers: None. A. Manavalan: None. Introduction: Pseudohyponatremia is a laboratory artifact that may result from elevated serum lipids, glucose, or proteins. We present a case of pseudohyponatremia secondary to severe elevation of lipoprotein X (Lp-X) in the setting of cholangiocarcinoma, where therapeutic plasma exchange (TPE) was used prophylactically as a bridge to definitive management. Case Presentation: A 73-year-old female with a history of diabetes, hypertension, and hyperlipidemia presented with jaundice. Initial workup revealed sodium 123 mEq/L, ALP 1239 U/L, AST 229 U/L, ALT 206 U/L, total bilirubin 11.1 mg/dL, and direct bilirubin 8.1 mg/dL, consistent with cholestasis. Evaluation of her hyponatremia demonstrated total cholesterol 998 mg/dL, LDL 907.8 mg/dL, HDL 17 mg/dL, and triglycerides 366 mg/dL; labs from one year prior: LDL 81 mg/dL, HDL 125mg/dL, and triglycerides 221mg/dL on statin monotherapy. Imaging uncovered a large necrotic mass obstructing the common hepatic duct bifurcation and the primary common bile duct, later confirmed to be cholangiocarcinoma. ERCP with attempted hepatic duct dilation was unsuccessful. The patient received a single prophylactic dose of TPE to prevent hyperviscosity syndrome (HVS) while awaiting external biliary drainage, resulting in significant improvement in LDL (359 mg/dL) and sodium levels (133 mEq/L). She was discharged on ursodiol and is currently undergoing oncologic management for her primary tumor. Discussion: This case illustrates pseudohyponatremia secondary to acute, severe hypercholesterolemia due to cholestasis. Lp-X elevation may occur in conditions such as cholestatic liver disease, post-transplant graft-versus-host disease, parenteral lipid-rich nutrition, and lecithin–cholesterol acyltransferase (LCAT) deficiency. Lp-X is difficult to measure directly but should be suspected in patients with these conditions who present with acute, marked hypercholesterolemia. Statins and ezetimibe are ineffective in lowering Lp-X levels. Although Lp-X lacks ApoB100 and is not atherogenic, high levels are associated with HVS. TPE is traditionally utilized for the management of HVS, however case studies support its use as prophylaxis until the cause can be identified and treated. In this case, initial failure to achieve biliary drainage, coupled with severely elevated LDL and pseudohyponatremia, prompted prophylactic TPE. Despite potential risks, it was safe and effective in reducing Lp-X levels while awaiting definitive treatment. Presentation: Saturday, July 12, 2025