An abnormal κ/λ sFLC ratio indicates residual disease in a proportion of IIMM patients classified as having achieved a complete response (CR) (Figure 18.4
. This has led to the incorporation of the κ/λ sFLC ratio in the definition of a stringent complete response (sCR) in the International Myeloma Working Group (IMWG) response criteria for MM 
. In addition to the criteria for a CR (absence of monoclonal protein by IFE and <5% clonal plasma cells (BMPCs) in bone marrow aspirates), a sCR requires the absence of BMPCs by immunohistochemistry and normalisation of the κ/λ sFLC ratio 
IMWG guidelines recommend that sFLCs are measured in all patients at suspected CR, in order to detect residual disease and therefore determine whether they have attained a sCR (Section 25.3.5) . Attainment of a sCR has prognostic significance, as it identifies patients with improved long-term outcome (Section 20.3.1) .