Pseudohypobicarbonataemia: A Rare Case of Enzymatic Assay Interference From Monoclonal Paraproteinaemia.

Pseudohypobicarbonataemia is a rare but important diagnostic pitfall that can lead to unnecessary investigations and inappropriate treatments if unrecognised. We present a case of a 92-year-old woman with chronic severe hypobicarbonataemia on enzymatic assay, despite normal acid-base parameters on blood gas analysis. Extensive evaluation excluded common causes of metabolic acidosis. Analytical studies identified interference by an IgG lambda paraprotein. Serial dilution showed a non-linear increase in bicarbonate concentration and polyethylene glycol precipitation improved bicarbonate recovery, implicating a protein-mediated effect. No turbidity or heterophile antibody interference was observed. This case highlights the importance of considering assay interference in patients with unexplained biochemical abnormalities to prevent misdiagnosis and unnecessary treatment. Additionally, it aims to encourage clinicians to utilise blood gas analysis as a reliable method to confirm bicarbonate levels when enzymatic assays are suspected to be inaccurate.