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Abstract: SA-PO475

Hyponatremia vs. Pseudohyponatremia: Sodium Measurement in a Patient With a Hematologic Malignancy

Session Information

Category: Fluid‚ Electrolyte‚ and Acid-Base Disorders

  • 1002 Fluid‚ Electrolyte‚ and Acid-Base Disorders: Clinical

Authors

  • Haeger, Sarah, University of Colorado Denver School of Medicine, Aurora, Colorado, United States
  • Roy, Nervik, University of Colorado Denver School of Medicine, Aurora, Colorado, United States
  • Young, Sarah Elizabeth, University of Colorado Denver School of Medicine, Aurora, Colorado, United States
Introduction

Hyponatremia is an electrolyte disturbance that can portend great morbidity. Once identified, hyponatremia must be confirmed as a true finding and not a laboratory artifact known as “pseudohyponatremia.” Delayed confirmation leads to a lag in treatment. Here we report a case of apparent pseudohyponatremia in a patient with chronic lymphocytic leukemia (CLL), subsequently found to have true hyponatremia due to the syndrome of inappropriate anti-diuretic hormone (SIADH).

Case Description

A 73 year old with a history of CLL presented with confusion and nausea. Initial laboratory tests revealed a white blood cell count of 71.4 x109 cells/L and a serum sodium concentration of 110mmol/L. Initial measured serum osmolarity was normal (284 mOsm/kg) raising concern for pseudohyponatremia. The patient had no paraprotein gap (total protein 6.0g/dL, albumin 3.3g/dL) and a normal triglyceride concentration (98 mg/dL). Repeat laboratory testing revealed true hyponatremia with a serum sodium concentration 108mmol/L, whole blood sodium concentration 108mmol/L, and serum osmolarity 227mOsm/kg. Further testing revealed a diagnosis of SIADH. CT scan revealed new lung nodules concerning for malignancy. The patient’s hospital course was prolonged due to refractory hyponatremia despite treatment with hypertonic saline, urea and tolvaptan.

Discussion

Serum sodium concentration, obtained on the basic metabolic panel, is routinely measured by indirect ion-selective electrode (ISE) methods which assumes that plasma contains 93% water. Serum sodium measurements using this method may be inaccurate when a patient has increased lipid, protein, or other non-aqueous substances in their blood. In contrast, whole blood sodium concentration is measured by direct ISE methods which does not make a plasma water content assumption. Serum osmolarity is used as a surrogate for whole blood sodium concentration. Serum osmolarity should be low in true hyponatremia, and if normal identifies pseudohyponatremia. We described a case where initial testing was misleading and delayed the diagnosis and treatment of true hyponatremia. For patients presenting with suspected hyponatremia, but with risk factors for pseudohyponatremia, we suggest repeat testing with serum sodium, whole blood sodium, and serum osmolarity to verify the diagnosis of pseudohyponatremia.