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

BACKGROUND: Major discrepancies between concentrations of serum total carbon dioxide (tCO) 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.

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

RESULTS: Mixing studies revealed non-linearity of serum tCO, suggesting the presence of interfering substances. Triglyceride concentrations were normal. Serum electrophoresis revealed a 0.4 mg/dl M-protein. The patient's serum tCO 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 tCO was significantly elevated after ultrafiltration of paraprotein, which confirmed the root cause of pseudohypobicarbonatemia.

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