1. What are Freelite assays?
Freelite® κ and λ free light chain (FLC) assays are quantitative, latex-enhanced immunoassays that are performed on routine turbidimetric or nephelometric instruments (Chapters 5 and 37). Freelite assays are based on polyclonal antisera directed against the “hidden” epitopes of immunoglobulin κ or λ FLC molecules that are located at the interface between the light and heavy chains of intact immunoglobulins (Section 5.1). These epitopes are only accessible when light chain molecules are not associated with the immunoglobulin heavy chain. κ and λ FLCs are measured in pairs to produce κ/λ FLC ratios to provide information about light chain clonality, or calculate the difference between the involved (iFLC) and uninvolved (uFLC) concentrations (dFLC, Section 7.2.2). Freelite assays can be run using serum, EDTA plasma, lithium heparin plasma, urine and CSF samples. Serum FLC (sFLC) measurements are included in international guidelines for the diagnosis, monitoring and prognosis of monoclonal gammopathies (Chapter 25).
2. How should I interpret the sFLC ratio?
For each serum sample, both κ and λ FLCs should always be measured, and the κ/λ ratio calculated by dividing the concentration of κ sFLCs by the concentration of λ sFLCs.
Freelite reference intervals were established by Katzmann et al.  using serum samples from 127 healthy donors aged 21 – 62 years, and 155 normal individuals aged 51 – 90 years (Section 6.1). A Freelite κ/λ sFLC ratio result within the 100% diagnostic range (0.26 – 1.65) indicates polyclonal sFLCs, whereas a value above or below the range indicates monoclonal κ or λ sFLC production, respectively.
Borderline ratios may occur in the absence of monoclonal gammopathy in patients with inflammatory conditions (e.g. infections, inflammation and autoimmune disorders, Section 6.2) or renal impairment, where the κ sFLC concentrations increase more than the concentrations of λ sFLCs and reflects their relative production rates (Section 6.3). A renal reference range for the κ/λ ratio (0.37 – 3.1) has therefore been established, which improves diagnostic specificity without affecting diagnostic sensitivity in patients with renal impairment (Section 6.3).
International Myeloma Working Group (IMWG) guidelines include sFLC analysis as part of the standard investigative workup of patients with suspected multiple myeloma (MM, Section 25.3.1). An involved/uninvolved Freelite sFLC ratio ≥100 is defined as a biomarker of malignancy in the diagnostic criteria for MM (Section 25.2.1), and the sFLC ratio should also be measured when patients achieve a complete response, to define a stringent complete response (sCR) (Section 25.3.5).