Since sFLC assays are intrinsically more sensitive than sIFE for detection of monoclonal FLC production (Chapter 4
), earlier detection of tumour relapse should be possible in IIMM patients who relapse with FLCs alone or potentially, a proportion of those who relapse with intact immunoglobulins and FLCs. In a study of 187 MM patients, Willenbacher et al. 
found that relapse was detected a median of 3 months earlier by sFLC levels than by conventional monoclonal protein measurements. Similar findings were reported by Dejoie et al. 
, who reported that of the 228 IIMM patients who progressed by SPE, 6.25% showed an increase in sFLCs that preceded the increase in intact immunoglobulin, by a median of 63 days. In a smaller study by Mösbauer et al. 
, relapse was identified earlier in eight of nine IIMM patients when sFLC ratios were used rather than IFE (median, 98 days; range 35 - 238 days) (Figure 18.9
In situations where relapse occurs relatively rapidly after successful treatment (i.e. within a few months), due to their relatively long half-life, monoclonal IgG levels may not normalise before relapse. Therefore, falling concentrations of IgG hide the early increases caused by tumour relapse. In contrast, because of their short half-life, an increase in sFLCs at relapse occurs from lower baseline levels and can be more readily detected. This is illustrated in a case study by Fuchida et al.  (Figure 18.10).