Routine evaluation of minimal residual disease in myeloma using next-generation sequencing clonality testing - feasibility, challenges and direct comparison with high sensitivity flow cytometry.

The 2016 International Myeloma Working Group consensus recommendations emphasize the importance of high sensitivity methods for minimal residual disease (MRD) detection, treatment response assessment and prognostication. Next generation sequencing (NGS) of IGH gene rearrangements is highly specific and sensitive, but its description in routine clinical practice and performance comparison with high sensitivity flow cytometry (hsFC) remain limited. In this large single institution study including 438 samples from 251 patients, we describe our use of NGS targeting the IGH and IGK genes for clonal characterization and monitoring, with comparison to hsFC. The index clone characterization success rate was 93.6% (235/251), which depended on plasma cell (PC) cellularity, reaching 98% when PC≥10% and below 80% when PC<5%. 85% of cases were successfully characterized using leader and FR1 primer sets, and most clones showed high somatic hypermutation rate (median 8.1%). Among monitoring samples from 124 patients, 78.6% (147/187) had detectable disease by NGS. Concordance with hsFC was 92.9% (170/183). Discordant cases encompassed 8/124 hsFC MRD+/NGS MRD- (6.5%) patients and 4/124 hsFC MRD-/NGS MRD+ (3.2%) patients, all with low-level disease near detection limits for both assays. Among concordant hsFC MRD-/NGS MRD- cases, only 5/24 patients (20.8%) showed subsequent overt relapse with 3-year follow-up. HsFC and NGS showed similar operational sensitivity, and the choice of test may depend on practical, rather than test performance, considerations.