8.6.3. Rationale for the diagnoses of LCMM missed by monoclonal antibody-based FLC assays

Chapter 8

The clinical studies discussed above have provided a number of examples where patients known to have LCMM have had normal N Latex FLC results but abnormal results with Freelite assays. There are two reasons why FLC assays may fail to identify the presence of monoclonal sFLCs: 1) antigen excess (Section 8.4.4), or 2) complete non-reaction, in which the monoclonal antibodies fail to recognise a particular FLC clone.

Pretorius et al. [175] performed a comparison of Freelite and N Latex FLC results for 116 samples sent to the laboratory for routine FLC analysis (after exclusion of samples with Freelite <50 mg/L), and investigated any samples that were highly discordant. For 6/116 (5.2%) samples, N Latex FLC assays gave markedly higher κ results (Figure 8.9). When these six samples were further diluted and retested using Freelite assays, the results significantly increased, consistent with antigen excess in the Freelite assays. In contrast, 4/116 samples (3.4%) had markedly higher λ Freelite results (Figure 8.9B). Further dilution did not increase the N Latex FLC λ results for these samples. These four patients included one with confirmed λ LCMM for whom the κ/λ sFLC ratio was normal by N Latex FLC assays, but abnormal by Freelite (Table 8.8).

It is noteworthy that the majority of diagnoses that were missed by the N Latex FLC assays were of λ type. Whilst the κ constant domain is typically encoded by a single C gene segment, the λ constant domain is encoded by one of a number of C gene segments (Section 3.3). It is probable that the monoclonal antibody-based assays fail to detect all polymorphic forms of FLCs, particularly λ FLCs that are more genetically diverse. Further evidence of the difficulties associated with raising monoclonal antibodies to recognise all λ light chain types was reported by Mollee et al. [993], and is discussed in Section 8.6.6.

A patient with λ LCMM who was not diagnosed by the N Latex FLC assays is presented as a clinical case study below. In this example, serum immunofixation confirmed the presence of monoclonal FLCs that were detected by the Freelite sFLC assays, but not the monoclonal antibody-based N Latex FLC assays.

Clinical case history

A patient with λ LCMM identified by Freelite but not N Latex FLC assays .

A 47-year-old woman was admitted to the Istituto Nazionale Tumori, Naples, with bone pain. An X-ray of her pelvis revealed osteolytic lesions, and a serum protein electrophoresis (SPE) was ordered but this revealed no obvious monoclonal protein. Subsequently, sFLC analysis was performed using both Freelite and N Latex FLC assays. Whilst the Freelite assay identified monoclonal λ sFLCs, the N Latex FLC assay results were normal (Table 8.9). High-resolution agarose electrophoresis of serum and urine samples identified a monoclonal protein band Figure 8.10A), which was typed by immunofixation electrophoresis (IFE) as monoclonal λ FLCs (in the absence of intact immunoglobulins including IgD/IgE; Figures 8.10B and C). A bone-marrow biopsy confirmed the diagnosis of λ LCMM. The patient was admitted to the Hematology-Oncology Unit, and one month after the start of therapy, serum and urine IFE became negative and the Freelite sFLC assay values returned to normal.

N Latex FLC
κ sFLC
9.1 3.3 - 19.4 16.4 6.7 - 22.4
λ sFLC
818.8 5.7 - 26.3 23.0 8.3 - 27.0
κ/λ sFLC
0.01 0.26 - 1.65 0.71 0.31 - 1.56

Table 8.8. Freelite and N Latex FLC patient results and reference intervals.