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FLC molecules are usually monomers or dimers, but higher polymeric forms frequently occur [181][182][183][73][185]. These act as multi-antigenic targets in immunoprecipitation assays, accelerating the formation of aggregates and leading to over-estimation of antigen concentrations (Figure 7.6). This occurs in some patients with nonsecretory multiple myeloma (NSMM) who have undetectable concentrations of sFLCs by IFE but can have high concentrations by nephelometry (Chapter 16).

In order to determine the effect of polymerisation on Freelite quantification, FLC monomers, dimers and polymers were purified from myeloma sera and concentrations compared with total protein measurements. It was apparent that purified dimers were over-estimated by 1.5-fold and higher polymers by 1.5- to 3.5-fold [186]. In a study by Émond et al. [183] greater than 7-fold over-estimation was observed in two samples (when compared with CZE) in association with polymers of up to 200 kDa.

An additional factor to consider is that SPE tests can underestimate FLC concentrations. Variable polymerisation may cause smearing of monoclonal bands on the gels so that only a proportion of the monoclonal protein is measured [186]. FLCs also take up less protein stain than albumin so their concentration is underestimated by scanning densitometry. It is likely that a combination of these factors causes the disparities between nephelometry and electrophoresis that are seen with the sera of some patients.

Whatever the explanation for the unexpectedly high results for samples containing polymerised sFLCs, it is important to note: 1) never try to disrupt polymers, treat as a normal sample; and 2) sFLC polymers are consistently produced so sFLC results provide valuable monitoring information and should be reported. In a patient with high sFLC results, if the patient responds to treatment the sFLC concentrations will reduce and the κ/λ sFLC ratio will eventually normalise in patients who achieve a stringent complete response (Chapter 25).

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