Heavy/Light-chain analysis of monoclonal gammopathies.

Monoclonal gammopathies (M proteins) are associated with an extraordinarily broad array of clinical conditions that range from monoclonal gammopathy of undetermined significance (MGUS) 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 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 to the region. They can be obscured by large proteins, such as haptoglobin in the 2 region or more commonly by transferrin and complement component 3 (C3) in the 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 light chains (FLCs) in the serum, the latter problem is disappearing (2 ). In addition to showing that an initial urine sample to rule out the presence of monoclonal FLCs is unnecessary, a large number of studies have demonstrated these antiFLC reagents to be of value for the initial assessment of prognosis for MGUS and multiple myeloma, detecting and following the monoclonal FLCs in patients with amyloid, detecting and following response to therapy in many cases of nonsecretory (oligosecretory) multiple myeloma, and detecting M proteins in patients with B-cell lymphoproliferative disorders (3–7 ). In this issue of Clinical Chemistry, Bradwell et al. (8 ) detail their development and deployment of reagent antisera with specificity for unique epitopes that form at the junction between the heavyand lightchain constant regions of each immunoglobulin molecule. Although some cross-reactivity remained in their current batches of specific anti–heavy/light-chain (HLC) reagents, the antisera created were specific enough for IgG , IgG , IgA , IgA , IgM , and IgM to establish a ratio for each HLC class in healthy blood bank donor sera and to demonstrate a potential use for these reagents in detecting and measuring the HLC molecules and their ratios in serum samples from patients with multiple myeloma. In comparing the sensitivity of HLC assays with the sensitivities of SPE and IFE, Bradwell et al. performed dilution studies on 9 sera containing M proteins. The end point for detection by electrophoresis was visualization by 3 experienced observers of a relatively low-resolution (5-band pattern) gel vs HLC results falling outside of the 95% CI. Under these conditions, HLC assays had a slightly lower analytical ability to detect IgG M proteins than SPE but had an equal or greater sensitivity for IgA and IgM M proteins (particularly those that comigrated with proteins in the region of their gel). Although, the authors reported no comparison dilution studies with IFE alone for sensitivity of detection, IFE and HLC results were concordant for 31 of the 40 SPE-positive or suspicious samples. A “minor” IgA by IFE was called an IgG by HLC assay, and Bradwell et al. suggested that this case might have reflected “IgG subclass dysregulation.” IFE detected 2 “minor” IgG bands that were not detected by HLC assay, and 4 samples with abnormal HLC results did not have M proteins according to IFE. The 1 Warde Medical Laboratory, Ann Arbor, MI. * Address correspondence to the author at: Warde Medical Laboratory, 5025 Venture Dr., Ann Arbor, MI 48108. Fax 734-665-0668; e-mail dfkeren@ yahoo.com. Received June 24, 2009; accepted June 26, 2009. Previously published online at DOI: 10.1373/clinchem.2009.132753 2 Nonstandard abbreviations: MGUS, monoclonal gammopathy of undetermined significance; SPE, serum protein electrophoresis; IFE, immunofixation; C3, complement component 3; FLC, free light chain; HLC, heavy/light-chain. Clinical Chemistry 55:9 1606–1608 (2009) Editorials