{"entity":{"id":"m-protein-free-light-chains","kind":"term","name":"M-protein, immunofixation and serum free light chains","aka":["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"],"tldr":"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.","asOf":"2026-09-17","wikipedia":"https://en.wikipedia.org/wiki/Myeloma_protein","links":[{"label":"Wikipedia","url":"https://en.wikipedia.org/wiki/Myeloma_protein"}],"tags":[],"related":["smoldering-myeloma","r-iss","mrd","umrd","flow-cytometry","tumour-markers","daratumumab","bortezomib","lenalidomide"],"cancers":["smouldering-myeloma","myeloma-transplant-eligible","myeloma-transplant-ineligible","myeloma-relapsed-refractory","plasma-cell-leukaemia","waldenstrom","marginal-zone-lymphoma"],"sections":[],"technologies":[],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":[],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"category":"Biomarkers"},"route":"/terms/m-protein-free-light-chains/","neighbours":{"term":[{"id":"flow-cytometry","kind":"term","name":"Flow cytometry (immunophenotyping)","route":"/terms/flow-cytometry/"},{"id":"mrd","kind":"term","name":"Minimal / molecular residual disease (MRD)","route":"/terms/mrd/"},{"id":"r-iss","kind":"term","name":"R-ISS / R2-ISS staging","route":"/terms/r-iss/"},{"id":"smoldering-myeloma","kind":"term","name":"Smouldering myeloma / MGUS","route":"/terms/smoldering-myeloma/"},{"id":"tumour-markers","kind":"term","name":"Tumour markers (CEA, LDH, chromogranin, thyroglobulin)","route":"/terms/tumour-markers/"},{"id":"umrd","kind":"term","name":"Undetectable MRD (uMRD / MRD-negative)","route":"/terms/umrd/"}],"drug":[{"id":"bortezomib","kind":"drug","name":"Bortezomib","route":"/drugs/bortezomib/"},{"id":"daratumumab","kind":"drug","name":"Daratumumab","route":"/drugs/daratumumab/"},{"id":"lenalidomide","kind":"drug","name":"Lenalidomide","route":"/drugs/lenalidomide/"}],"cancer":[{"id":"marginal-zone-lymphoma","kind":"cancer","name":"Marginal zone lymphoma","route":"/cancers/marginal-zone-lymphoma/"},{"id":"myeloma-transplant-eligible","kind":"cancer","name":"Newly diagnosed multiple myeloma, transplant-eligible","route":"/cancers/myeloma-transplant-eligible/"},{"id":"myeloma-transplant-ineligible","kind":"cancer","name":"Newly diagnosed multiple myeloma, transplant-ineligible","route":"/cancers/myeloma-transplant-ineligible/"},{"id":"plasma-cell-leukaemia","kind":"cancer","name":"Plasma cell leukaemia","route":"/cancers/plasma-cell-leukaemia/"},{"id":"myeloma-relapsed-refractory","kind":"cancer","name":"Relapsed or refractory multiple myeloma","route":"/cancers/myeloma-relapsed-refractory/"},{"id":"smouldering-myeloma","kind":"cancer","name":"Smouldering multiple myeloma","route":"/cancers/smouldering-myeloma/"},{"id":"waldenstrom","kind":"cancer","name":"Waldenström macroglobulinaemia","route":"/cancers/waldenstrom/"}]}}