Table 10.
Serum free light chain immunoassay properties.
| Property | Comment |
|---|---|
| Precision and lot-to-lot variability of reagent | Variability between reagent lots should be checked by repeat sample assay or use of a normal/elevated serum-based control.56,59,70,71 |
| Antibody specificity | Cross reactivity with bound light chains and intact immunoglobulin should be checked to avoid overestimation of serum FLC. False negative results occur due to limited reactivity (conformation of FLC) to anti-FLC antibody or because of a lack of antigen recognition (large monoclonal FLC diversity).56,72 |
| Calibration traceability | No reference measurement system (reference method or reference material) is available for serum FLC. Preliminary work suggests FLC agree more closely with serum protein electrophoresis values than with immunoassay values.73 Despite some traceability of assay calibrators to the predicate assay (Freelite®) polyclonal calibrants, absolute FLC values and ratios may differ between manufacturers’ assays.15,43,70 |
| Antigen excess | Serum FLC measurement spans the concentration range from ~1 to 10,000–20,000 mg/L with the potential for antigen excess to occur in 2-site sandwich immunoassays and underestimation of FLC concentration.69 |
| Nonlinearity | Nonlinearity may occur at relatively low FLC concentrations due to limited reactivity with antibody or blockage of reactive sites on the FLC molecule by other proteins.59,74 |
| Nephelometric overestimation of FLC | Multimers may be present and over-react in immunonephelometric assays e.g. NSMM with a 1.5–3.5-fold overestimation compared to other MM sera.75 They are artefactual but, despite this, FLC can be used to monitor disease response. |
| Gap effect | At the lower and upper extremes of the Freelite® measuring range serum FLC underestimation occurs resulting in inaccurate FLC ratio.59 |
| Differences in absolute FLC values and ratios between assays | Absolute FLC values and ratios may differ between manufacturers’ assays and between different platforms (using Freelite® assay).56 FLC ratios may be inaccurate at very low, suppressed FLC concentrations. It is suggested to use ‘difference in FLC’ (dFLC) which is involved FLC minus uninvolved FLC to monitor disease response. |
FLC, free light chain(s); MM, multiple myeloma; NSMM, nonsecretory myeloma.