For each sample, both κ and λ sFLCs should always be measured, and the κ/λ sFLC ratio calculated. Some centres (especially those taking part in clinical trials) may also wish to calculate the difference (dFLC) between the involved (iFLC) and uninvolved (uFLC) sFLCs. Definitions of other common terms/abbreviations are summarised in Table 7.3.

Term Definition Comment For a κ FLC tumour
iFLCInvolved FLC The FLC type that is produced by the tumour κ
uFLCUninvolved FLC The FLC type that is the alternate light chain type to the iFLC
(not to be confused with urine FLCs)
κ/λ sFLC ratioκ/λ A ratio of the concentration of κ to λ sFLCs κ/λ
dFLCiFLC - uFLC The difference in the concentration between the iFLC and uFLC κ - λ
ΣFLCSummated FLC The sum of κ and λ sFLC concentrations, determined by two separate assays κ + λ

Table 7.3. Summary of FLC terminology.

International Myeloma Working Group (IMWG) guidelines for sFLC analysis in multiple myeloma (MM) and related disorders recommend that the κ/λ sFLC ratio is calculated as part of the standard investigative workup of patients with suspected multiple myeloma (Chapter 25) [167]. The ratio is now included in IMWG criteria for the diagnosis of multiple myeloma: an involved/uninvolved Freelite sFLC ratio ≥100 is designated a biomarker of malignancy (Section 25.2.1), and it is also important when documenting a stringent complete response (Section 25.3.5).

For serial measurements, either the iFLC or the dFLC is recommended [167]. There are two principal reasons for this: 1) small changes in uFLC concentrations produce large changes in the sFLC ratio, even when the iFLC is unchanged. Therefore, changes in the κ/λ sFLC ratio may not always be a surrogate marker for tumour burden; and 2) using dFLC can remove some of the variation seen in iFLC values due to changes in renal function.