B-350 Diagnostic performance of the EXENT® System in monoclonal gammopathy

Abstract Background The Immunoglobulin Isotypes (GAM) Assay for the EXENT Analyser (The Binding Site, part of Thermo Fisher Scientific) offers sensitive detection, isotyping and quantification of M-proteins. We aimed to assess the clinical performance of the EXENT System (The EXENT Analyser and the Immunoglobulin Isotypes (GAM) Assay for the EXENT Analyser) as an aid in diagnosis of multiple myeloma (MM) and associated disorders. Methods The study included 1135 patients: 718 with monoclonal gammopathy (215 MM, 152 smoldering MM (SMM), 243 monoclonal gammopathy of undetermined significance (MGUS), 53 Waldenström’s Macroglobulinemia (WM), and 55 AL amyloidosis), and 417 disease controls. Clinical diagnoses were defined per appropriate international guidelines. Except for 20 AL amyloidosis patients, all monoclonal gammopathy patients were from the US population. The monoclonal gammopathy cohort had a 1:1 male/female ratio and was ethnically diverse (341 White; 264 African American/ Black; 43 Asian; 17 Hispanic; 53 unknown). The M-protein was IgG in 359 patients, IgA in 95, IgM in 97, light chain-only in 89, IgD in 2; 64 patients had more than one M-protein, and no M-protein was reported by electrophoretic methods in 9 cases. Two IgA, and one IgM patient had heavy chain identified only. In the disease control patients, the diagnosis of monoclonal gammopathy had been ruled out. Serum samples were analyzed at three testing sites using the EXENT System. Diagnostic sensitivity and specificity were calculated based on categorizing results as positive or negative with the EXENT System. A positive result was defined as the presence of an M protein which was either an intact immunoglobulin (IgG/=0.359 g/L; IgA/=0.325 g/L; IgM/=0.227 g/L) or a light chain-only. These isotype-specific cut-off values were established on samples from 364 apparently healthy US subjects for IgG, IgA and IgM M-proteins using the 95th percentile limit for IgG, and the 99th percentile limit for IgA and IgM. A negative result was defined as no M-protein by the EXENT System; or if intact immunoglobulin M-proteins were present, the concentration was below the corresponding isotype-specific cut-off. Diagnostic sensitivity and specificity with 95% confidence interval (CI) were calculated for each diagnosis (MGUS, SMM, MM, WM and AL amyloidosis) separately, and on the pooled results. Results The overall diagnostic sensitivity for monoclonal gammopathies was 92.6% (CI: 90.5-94.3%). Specifically, diagnostic sensitivity was 90.5% (86.2-93.6%) for MGUS; 100.0% (97.5-100.0%) for SMM; 90.2% (85.5-93.5%) for MM; 100.0% (93.2-100.0%) for WM; and 83.6% (71.7-91.1%) for AL amyloidosis. The diagnostic specificity of the assay was 76.5% (72.2-80.3%). The EXENT System identified an M-protein in more patients than serum protein electrophoresis (SPE): 665 (92.6%) vs 611 (85.1%). The positivity rate was 90.5% vs 88.1% in MGUS; 100% vs 92.8% in SMM; 90.2% vs 80.5% in MM; 100% vs 100% in WM; and 83.6% vs 54.5% in AL amyloidosis. Conclusion The EXENT System demonstrated the same or superior diagnostic sensitivity compared to SPE. Whilst the assay is not recommended as a first-line screening test, this clinical performance suggests that it has the potential to become part of the diagnostic algorithm in the workup for monoclonal gammopathies.