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Cryoglobulins are serum immunoglobulins that reversibly precipitate at temperatures below 37°C. Cryoglobulinaemia is classified according to the clonality and rheumatoid factor activity of the cryoglobulin [736]. It is associated with many illnesses, which can be broadly grouped into infections, autoimmune disorders, and malignancies. The most common cause is infection with HCV [737].

sFLC concentrations are elevated in many patients with monoclonal cryoglobulinaemia, and may serve as a useful tool for monitoring response to treatment since direct measurement of cryoglobulins is technically difficult. Pattenden et al. [711] described a case of Waldenström’s macroglobulinaemia associated with a type 1 cryoglobulin. Serum protein electrophoresis (SPE) revealed the presence of an IgMλ monoclonal protein that could not be quantitated since it precipitated out of the gel. However, serial sFLC measurements were successfully used to monitor response to chemotherapy (Chapter 32, Clinical Case History). Besada et al. [738] report a case of type-1 (IgGκ) cryoglobulinaemic vasculitis (treated successfully with bortezomib after rituximab failure) that was effectively monitored with sFLCs.