Heavy/Light-Chain Analysis of Monoclonal Gammopathies

Abstract

Monoclonal gammopathies (M proteins) are associated with an extraordinarily broad array of clinical conditions that range from monoclonal gammopathy of undetermined significance (MGUS)1 to multiple myeloma. Three basic goals of the methods used to investigate M proteins are detection, immunochemical characterization, and quantification. Serum protein electrophoresis (SPE) techniques have undergone considerable improvements in the past few decades and now provide crisp resolution (especially in the β region) to detect many subtle electrophoretic differences in M proteins not readily distinguished by older 5-band gel techniques. Immunofixation (IFE) has replaced immunoelectrophoresis to become the gold standard for detecting an M protein and characterizing its isotype. Despite these advances, however, problems in detecting and especially in quantifying M proteins persist. By SPE, M proteins can migrate anywhere from the α to the γ region. They can be obscured by large proteins, such as haptoglobin in the α2 region or more commonly by transferrin and complement component 3 (C3) in the β region (1). Occasionally, IgM M proteins self-aggregate, creating problems with detection and measurement. Although most cases of multiple myeloma feature relatively large M protein peaks that are readily detected, even with low-resolution 5-band methods, 15%–20% of patients with multiple myeloma produce only free immunoglobulin light chains that, because of their relatively small molecular weights, are rapidly cleared into the urine, often producing no detectable M protein spike after SPE. Indeed, many cases of monoclonal free light chains (Bence Jones protein) cannot be identified by IFE of serum; however, IFE of concentrated early morning void (or 24-h urine) remains a reliable means to detect such small molecular weight M proteins. Unfortunately, urine samples too often are not submitted along with serum samples for the initial evaluation. With the development by Bradwell et al. in 2001 of highly specific assays for free …