Paraprotein Interference in Routine Chemistry: A Case Report of Spuriously Elevated Bilirubin

Abstract

Paraprotein interference in routine chemistry assays is an under-recognised cause of a spurious result that does not match the
clinical picture, and bilirubin assays are among those susceptible to it through turbidity and direct spectral interference from a
high concentration of monoclonal immunoglobulin. We report a case of a 68-year-old man with known IgM monoclonal
gammopathy who presented with a total bilirubin of 187 μmol/L on routine chemistry despite normal liver function otherwise,
normal conjugated bilirubin by an alternative method, and no clinical or ultrasound evidence of hepatic or biliary disease. Serial
dilution of the sample produced a non-linear response inconsistent with a true bilirubin elevation, and repeat measurement by a
diazo method on an ultracentrifuged, paraprotein-depleted sample returned a bilirubin of 14 μmol/L, within the reference range.
We describe the interference mechanism, the clues that distinguished it from genuine hyperbilirubinaemia, and a practical
laboratory pathway for the discordant bilirubin result in a patient with known or suspected paraproteinaemia.