Although most cases of DLBCL are light chain restricted, only a minority of patients have sFLC abnormalities. Between 9 and 14% of DLBCL patients have an abnormal κ/λ sFLC ratio and between 19 and 32% have elevated κ and/or λ sFLCs [662][663]. Of the patients with elevated sFLCs, approximately 25% have monoclonal sFLCs (with an abnormal κ/λ sFLC ratio) while the remainder have polyclonal sFLCs (with a normal κ/λ sFLC ratio) [664].

Jardin et al. [663] characterised serum IgM, IgG and IgA HLC in DLBCL. An abnormal IgMκ/IgMλ, IgGκ/IgGλ or IgAκ/IgAλ HLC ratio was observed in the sera of 19.1%, 6.4% and 2.9% of patients, respectively [663]. This is consistent with the relative frequencies of B-cell receptor isotype expression by DLBCL cells [695]. The most frequent HLC abnormalities were reduced serum concentrations, observed in 11.2% (IgMκ) to 21% (IgGκ) of patients. There was little overlap between patients with sFLC and IgM HLC abnormalities (Figure 31.4 ).

Patients with an abnormal IgMκ/IgMλ HLC ratio had an increase in Cμ mRNA expression and a higher rate of IgM positive cases by immunohistochemistry compared with patients with normal HLC ratios [663]. Therefore, in the absence of any coexistent monoclonal gammopathies (e.g. monoclonal gammopathy of undetermined significance, Chapter 13), the source of a monoclonal protein in DLBCL is likely to be the tumour.