A rare case of persistent pseudohypobicarbonatemia arising from chemistry analyzer-specific interference

Major discrepancies between concentrations of serum total carbon dioxide (tCO₂) obtained from chemistry analyzers and calculated bicarbonate from blood gas analyzers should prompt laboratory investigation. Here, we present a rare case of pseudohypobicarbonatemia unrelated to the common causes such as hypertriglyceridemia and hyperproteinemia, but was caused by a low concentration of paraproteins.

A 75-year-old man with persistent fevers was found to have a low concentration of serum tCO₂ (<10 mmol/l) with a normal pH and calculated bicarbonate concentrations (23.5 mmol/l) from the blood gas analyzer. His serum tCO₂ concentrations remained critically low throughout hospitalization without any evidence of acidosis. Serum tCO₂ levels were measured via Siemens ADVIA Chemistry XPT system.

Mixing studies revealed non-linearity of serum tCO₂, suggesting the presence of interfering substances. Triglyceride concentrations were normal. Serum electrophoresis revealed a 0.4 mg/dl M-protein. The patient's serum tCO₂ concentrations were repeated on different chemistry analyzer platforms - including Siemens, Roche, and Abbott - which demonstrated that the interference was specific to the Siemens chemistry analyzer. Serum tCO₂ was significantly elevated after ultrafiltration of paraprotein, which confirmed the root cause of pseudohypobicarbonatemia.

Laboratory professionals should be aware that spuriously low serum tCO₂ concentrations may result from unique interfering substances, such as paraproteins, that are both patient- and chemistry analyzer-specific.