# M-protein, immunofixation and serum free light chains

Source: https://onco.cc/terms/m-protein-free-light-chains/  
OnCo record `m-protein-free-light-chains` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Myeloma cells are clones of one antibody-making cell, so they pour a single identical antibody, the M-protein, into the blood; measuring it (and the free light chains that go with it) is how myeloma is diagnosed, how deep a response is judged, and how relapse is caught before symptoms.

## Summary

What is measured: the monoclonal immunoglobulin made by a plasma cell clone, and its unpaired light chains. How: serum protein electrophoresis quantifies the monoclonal peak in g/L; immunofixation identifies the heavy chain (IgG, IgA, IgM, rarely IgD or IgE) and light chain (kappa or lambda); the serum free light chain assay reports kappa, lambda and their ratio (normal 0.26 to 1.65); 24-hour urine electrophoresis and immunofixation find Bence Jones protein. Diagnostic thresholds: MGUS under 30 g/L, smouldering myeloma 30 g/L or more or urine protein of 500 mg a day without myeloma-defining events, and an involved to uninvolved free light chain ratio of 100 or more is itself a myeloma-defining event; light-chain-only myeloma is about 15 percent and non-secretory disease about 3 percent, needing marrow and imaging; Waldenström's has an IgM paraprotein and viscosity testing, AL amyloidosis is followed by free light chains, and daratumumab (an IgG kappa antibody) can mimic a small M-protein unless a shift assay is used. What a result changes: the IMWG response categories run from partial (a fall of half or more), very good partial (90 percent or more, or immunofixation-positive only), complete (negative immunofixation with under 5 percent plasma cells) to stringent complete (normal ratio, no clonal cells) and then measurable residual disease; progression is a 25 percent rise with an absolute increase of 5 g/L; values are checked every one to three months on therapy and every three to six months in remission, and a rising paraprotein triggers imaging and marrow before symptoms return. Where it matters: smouldering myeloma, transplant-eligible and ineligible myeloma, relapsed myeloma, plasma cell leukaemia, Waldenström's and marginal zone lymphoma.

## Fields

- Kind: Term
- Last checked: 2026-09-17
- Also known as: M-protein; M protein; M-spike; monoclonal protein; paraprotein; serum protein electrophoresis; SPEP; urine protein electrophoresis; UPEP; immunofixation; IFE; serum free light chains; sFLC; free light chains; free light chain; serum free light chain; free light chain ratio; FLC ratio; involved free light chain; kappa lambda ratio; light chain; light chains; Bence Jones protein; light-chain myeloma; IgM paraprotein; M-protein response; paraprotein response

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Myeloma_protein
- Wikipedia: https://en.wikipedia.org/wiki/Myeloma_protein

## Connected records

- terms: [Flow cytometry (immunophenotyping)](https://onco.cc/terms/flow-cytometry/), [Minimal / molecular residual disease (MRD)](https://onco.cc/terms/mrd/), [R-ISS / R2-ISS staging](https://onco.cc/terms/r-iss/), [Smouldering myeloma / MGUS](https://onco.cc/terms/smoldering-myeloma/), [Tumour markers (CEA, LDH, chromogranin, thyroglobulin)](https://onco.cc/terms/tumour-markers/), [Undetectable MRD (uMRD / MRD-negative)](https://onco.cc/terms/umrd/)
- drugs: [Bortezomib](https://onco.cc/drugs/bortezomib/), [Daratumumab](https://onco.cc/drugs/daratumumab/), [Lenalidomide](https://onco.cc/drugs/lenalidomide/)
- cancers: [Marginal zone lymphoma](https://onco.cc/cancers/marginal-zone-lymphoma/), [Newly diagnosed multiple myeloma, transplant-eligible](https://onco.cc/cancers/myeloma-transplant-eligible/), [Newly diagnosed multiple myeloma, transplant-ineligible](https://onco.cc/cancers/myeloma-transplant-ineligible/), [Plasma cell leukaemia](https://onco.cc/cancers/plasma-cell-leukaemia/), [Relapsed or refractory multiple myeloma](https://onco.cc/cancers/myeloma-relapsed-refractory/), [Smouldering multiple myeloma](https://onco.cc/cancers/smouldering-myeloma/), [Waldenström macroglobulinaemia](https://onco.cc/cancers/waldenstrom/)

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