Freelite assays are the only FLC assays recommended by name in International Myeloma Working Group guidelines, and an involved/uninvolved Freelite sFLC ratio ≥100 is designated as a biomarker of malignancy (Chapter 25) . Guidelines and response criteria that are based on Freelite data should not be applied on the basis of N Latex FLC results because of the poor concordance between the absolute values . A summary of clinical definitions that are based upon quantitative Freelite results are shown in Table 8.7.
|AL amyloidosis ||Measurable disease||dFLC >50 mg/L|
|VGPR||dFLC <40 mg/L|
|MM ||Measurable disease||100 mg/L|
|Progressive disease||25% increase in dFLC from lowest confirmed response (provided absolute increase >100 mg/L)|
Table 8.7. A summary of international guidelines based on quantitative Freelite results. VGPR: very good partial response; dFLC: the difference between involved and uninvolved sFLC concentrations. compared the assessment of response indicated by N Latex and Freelite FLC assays for 42 MM patients who had been treated after relapse. The response criteria used were those defined by Rajkumar et al. . A total of 17/42 patients had measurable disease by both assays and 11 by neither; one patient was evaluable by N Latex FLC only but 13 had measurable disease by Freelite only. For the 17 patients with measurable disease by both assays, there was poor agreement in the assigned response to treatment (Figure 8.9, weighted Kappa 0.75).
Dejoie et al.  compared the ability of N Latex and Freelite FLC assays to identify disease progression in LCMM patients according to IMWG guidelines. At clinical progression, Freelite confirmed progressive disease in 15/19 patients, compared to 11/19 by N Latex FLC assays. This study further supports the recommendation that IMWG guidelines based on Freelite data cannot be applied to the monoclonal assays.