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It is reasonable to assume that any assay calibrated against another assay should produce similar quantitative results. However, it has been shown that this is not the case for the N Latex FLC and Freelite assays, and it cannot be assumed that N Latex FLC assays will have the same clinical utility or give compliance with current guidelines (Chapter 25). Of particular concern is the number of clinically confirmed diagnoses of LCMM that have been missed by the N Latex FLC assays. This is most likely due to the failure of the monoclonal antibodies to recognise the full repertoire of polymorphic monoclonal FLCs.

Questions

  1. Can an individual monoclonal antibody directed against FLC epitopes be used to produce a turbidimetric/nephelometric FLC immunoassay?
  2. Are κ/λ sFLC ratios, measured with N Latex FLC assays, found to be higher in patients with severe renal impairment (CKD5)?
  3. Are international guidelines applicable to N Latex FLC assays?

Answers

  1. No. Lack of cross linking between the antibody and FLC antigen fails to produce the large immune complexes that are required to scatter light (Section 8.3.3).
  2. No. In fact λ sFLC concentrations measured by N Latex FLC assays are relatively higher in CKD5, resulting in a κ/λ sFLC ratio that is significantly lower than healthy controls. It has been suggested this may be an analytical artefact due to a low molecular weight interfering substance (Section 8.5.2).
  3. No. Absolute values reported by N Latex FLC and Freelite sFLC assays do not compare well (Section 8.5.3), and N Latex FLC assays fail to identify monoclonal sFLCs in cases of LCMM that are detected by Freelite assays (Section 8.5.5). Therefore, it cannot be assumed that International guidelines (that are based on quantitative Freelite data) are applicable to the monoclonal antibody-based assays (Section 8.5.4 and Chapter 25).
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