Torti et al.  performed a retrospective analysis of 180 newly diagnosed MM patients and compared the use of traditional CRAB markers with the updated IMWG criteria at diagnosis. The majority of cases had a previous history of MGUS and SMM. A total of 21% of patients had at least one detectable SLiM biomarker before the clear manifestation of CRAB criteria; a highly abnormal sFLC ratio was the most common finding, being present in two-thirds of cases. Patients with SLiM features were at a very high risk of progression, with 92% developing CRAB features and progressing to active MM in less than one year. This study confirms the ability of the new MM diagnostic criteria to identify patients who are likely to benefit from early treatment before the development of end-organ damage.. The majority (85%) of patients with MM secrete an intact immunoglobulin monoclonal protein. The relative frequency of the classes of immunoglobulin secreted by the plasma cell clones reflect the frequencies normally observed in the body. Light chain MM (LCMM) and nonsecretory MM (NSMM) represent a further 13% and 2% of MM patients, respectively. A summary of MM monoclonal protein types is shown in Figure 12.5; this data is from more than 2500 patients entered into the UK Medical Research Council MM trials .
Over the past 15 years, there have been publications covering many aspects of sFLC analysis in the diagnosis, monitoring and prognosis of MM; these are reviewed in individual Chapters covering LCMM (Chapter 15), NSMM (Chapter 16) and intact immunoglobulin MM (IIMM; Chapters 17 and 18). Additionally, clinical applications of sFLC testing have been described in other malignant conditions that are related to MM, including plasmacytoma (Chapter 21) and plasma cell leukaemia (Chapter 22). The inclusion of sFLCs for routine monoclonal gammopathy screening and the relative merits of serum versus urine FLC measurements are also discussed (Chapter 23 and Chapter 24). It is well established that sFLC measurements provide valuable information in patients with diseases with monoclonal light chain deposition, such as cast nephropathy, AL amyloidosis and light chain deposition disease. These clinical applications are addressed in the succeeding section (Section 5).
Following the introduction of Hevylite assays, there is accumulating evidence showing that HLC analysis provides an additional tool for the management of patients with MM, by giving a quantitative and non-subjective indication of clonality. This evidence is presented in Chapters 17 and 18.