Pseudohypobicarbonatemia in a patient with amyloidosis

We report a case of a patient who had critically low serum bicarbonate (HCO₃ ^-) levels ranging from 8 to 11 mmol/L on repeated venous measurements using an enzymatic/photometric assay. This prompted hospitalization and treatment with intravenous sodium bicarbonate (NaHCO₃) followed by oral NaHCO₃. He was evaluated for potential causes of high anion gap metabolic acidosis without any etiology found. He continued to have low serum HCO₃ ^- levels despite maintenance oral NaHCO₃ therapy and was referred for a second opinion where further laboratory work was pursued. An arterial blood gas was obtained, which revealed normal whole blood pH and HCO₃ ^- levels. A different enzymatic/photometric assay revealed a normal serum HCO₃ ^- level at 21 mmol/L. Additional workup revealed paraproteinemia, which was thought to interfere with the enzymatic process by which his serum HCO₃ ^- was measured, resulting in erroneous values.