The introduction of the serum free light chain (sFLC) changed the diagnostic paradigm for patients with B cell disorders. IMWG guideline recommends the assay as a replacement for 24h urine at diagnosis, however with the exception of oligosecretory disease the assay is not recommended as a tool to monitor patients. One rationale for this recommendation is that to date, studies have compared the concentrations of FLC as measured by the two tests rather than determine which test provides the more reliable clinical assessment. Here we compare the sensitivities of FLC and 24h urine and comment on the reliability of each to monitor patients.

Sequential sera from 25 LCMM (14 FLCκ, 11 FLCλ; stage I: 10, II: 10, III: 5) and 157 IIMM patients (79 IgGκ, 34 IgGλ, 26 IgAκ, 18 IgAλ; Stage I: 46, II: 75, III: 35, 1 missing) enrolled onto the IFM 2007-02 MM trial were analysed. Serum FLCκ and FLCλ levels were measured by Freelite® in samples collected at presentation, after cycles 2 and 4 of therapy and post ASCT. Results were compared to previously published sFLC reference ranges (sFLCκ 3.3-19.4 mg/L, sFLCλ 5.7-26.3 mg/L, sFLCκ/λ ratio 0.26-1.65), SPEP, UPEP sIFE and uIFE. IMWG guidelines were used to define measurable disease and to assess response the therapy. Quadratic Weighted Kappa (WK) analysis was performed to assess agreement in responses assigned by sFLC and urine tests.

All 25 LCMM patients had abnormal sFLC ratios (14 FLCκ, 11 FLCλ) and measurable disease at presentation (iFLC 3620 (689-22000) mg/L). Similarly, all patients were positive by uIFE and had measurable disease by UPE (1940 (490-42000) mg/24h). However, in keeping with previous reports quantitative correlation between the two assays was poor (r=0.27).

Responses assigned by sFLC and UPEP were concordant in 11/25 (44%) patients, although in 4/11 (40%) timing of the response was different (UPEP 89 (58-118) days; sFLC 226 (216-227) days). In the remaining 14/25 patients the responses assigned using the two tests differed. In 7/14 patients UPEP and uIFE became negative whilst the FLC ratio remained abnormal; in 1/7 patient sIFE confirmed the presence of the M protein. In 3 patients FLC identified relapse, while UPEP was negative or indicated response. In a further 2 patients FLC identified no response whilst UPEP initially identified a response and subsequent relapse. Overall, a moderate concordance was identified between the responses assigned by sFLC and urine tests (WK (95% CI): 0.59 (0.36-0.82)).

At presentation, 154/157 (98%) IIMM patients had abnormal FLC ratios (FLCκ ratio 57 (2-33191); FLCλ ratio 0.009 (0.00003-0.25)), whereas only 85/157 (54%) patients were positive by uIFE and 67/157 (43%) by UPEP. 98/157 (62%) had measurable disease using sFLC (κFLC 491 (101-15600) mg/L; λFLC 441 (101-14100) mg/L), and 55/157 (35%) had measurable disease by UPEP (1000 (210-9200) mg/24h). 53/157 (34%) patients had measurable disease by both methods. The correlation between sFLC and UPEP measurements was poor (r=0.36) as was the correlation between intact immunoglobulin measurements by SPEP and sFLC (r=-0.06) or UPEP (r=-0.26).

In 53 IIMM patients with measurable disease by both FLC and UPEP, sFLC ratios normalised in 14/53 patients (in 8/14 sIFE remained positive) while uIFE became negative in 33/53 patients (20/33 remained sIFE positive). WK showed better agreement for response assignment between intact immunoglobulin and sFLC measurements (WK (95% CI): 0.63 (0.48-0.79); substantial agreement) than with urine tests (0.49 (0.27-0.72); moderate agreement). Additionally, there was an association between depth of response by sFLC pre- and post-transplant: patients achieving >VGPR before transplant were more likely to achieve >VGPR post-transplant compared with patients who achieved <VGPR prior to transplant (96.2% vs. 63.2%, respectively; p=0.001).

Finally 5/157 IIMM patients were oligosecretory and had measurable levels of disease by both UPEP and sFLC, but not by SPEP. In all 5 patients UPEP became negative by cycle 2; however, an abnormal sFLC ratio and positive sIFE indicated persistent disease.

sFLC was a more sensitive tool and showed a greater degree of concordance with IFE and SPEP than UPEP in LCMM and IIMM patients respectively during patient monitoring. Furthermore, >90% reduction in sFLC prior to transplant was associated with post-transplant response in IIMM patients. Larger studies with patient outcome are required to validate our findings.


No relevant conflicts of interest to declare.

Author notes


Asterisk with author names denotes non-ASH members.