The prognostic utility of baseline sFLC measurements was evaluated by Dingli et al. . A total of 43/116 patients progressed to MM with a median time to progression of 1.8 years. An abnormal κ/λ sFLC ratio was associated with a 44% risk of progression at 5 years compared with a 26% risk in patients with a normal κ/λ sFLC ratio (p=0.039) (Figure 21.2). Patients with an abnormal κ/λ sFLC ratio also had a shorter overall survival (Figure 21.3). At 1 - 2 years after therapy, a persistent serum monoclonal immunoglobulin concentration of ≥5 g/L was an additional risk factor for progression to MM. A risk stratification model was therefore constructed based on these two risk factors (an abnormal κ/λ sFLC ratio at baseline and a monoclonal protein concentration of >5 g/L at 1 - 2 years following diagnosis). Low-, intermediate- and high-risk groups of progression to MM corresponded to none, one or two risk factors, and these gave 5-year progression rates of 13%, 26% and 62% respectively (Figure 21.4). The authors commented that sFLC analysis provided an important prognostic indicator in these patients.
Subsequent studies have further highlighted the prognostic potential of sFLC analysis. Koch et al.  reported that 17/32 (53%) patients with an abnormal sFLC ratio at diagnosis progressed to MM compared with 2/17 (12%) patients with a normal κ/λ sFLC ratio at diagnosis. The authors concluded that an abnormal baseline κ/λ sFLC ratio was significantly associated with progression to MM (p=0.012). Most recently, Fouquet and colleagues  proposed a risk stratification model that incorporates an abnormal involved FLC (iFLC) concentration and whole body, fluorodeoxyglucose positron emission tomography – computed tomography (FDG-PET CT) at diagnosis. This model was constructed using patients with both bone and extramedullary plasmacytomas and is discussed in detail in Section 21.3.
In summary, these reports indicate that baseline sFLC abnormalities are consistently associated with increased risk of progression from SPB to MM, and sFLC analysis is recommended in IMWG guidelines for all patients with solitary plasmacytoma (Section 25.3.1).