Interference of Monoclonal Antibody Therapies with Serum Protein Electrophoresis Tests

To the Editor: During routine serum protein electrophoresis (SPE), we recently detected an apparent IgG heavy-chain immunoglobulin in a 51-year-old woman with a 5-year history of IgDκ multiple myeloma. Other relevant laboratory values at the time included the following: serum creatinine, 159 μmol/L (reference interval, 62–97 μmol/L); IgG, 2.55 g/L (reference interval, 6.00–17.00 g/L); IgM, <0.25 g/L (reference interval, 0.35–2.90 g/L); and IgA, 0.40 g/L (reference interval, 0.40–4.00 g/L). The rarity of an IgG heavy-chain gammopathy in combination with an IgDκ and a low quantitative IgG value raised suspicion of an interference. Consultation with the treating physician revealed that the patient was enrolled in a phase II clinical trial of dexamethasone and siltuximab for the treatment of refractory myeloma. Siltuximab (Centocor), a high-affinity antibody to human interleukin-6, is currently being used in clinical trials for the treatment of a number of malignancies, including multiple myeloma. Siltuximab is a chimeric immunoglobulin comprising the variable antigen-binding region of a mouse antibody and the constant region of human IgG1κ (1). It binds to human interleukin-6, a survival factor for myeloma cells (2). We obtained siltuximab from the manufacturer …