Serum Free Light Chains

Plasma cells make immunoglobulin light chains in excess of heavy chains, and the surplus appears in serum as free kappa and free lambda light chains. The serum free light chain (sFLC) assay is a nephelometric or turbidimetric immunoassay using antibodies that react only with epitopes hidden in intact immunoglobulin, so it measures free kappa, free lambda and their ratio.

A clonal plasma cell or B cell disorder skews the ratio, because one chain type is overproduced. The assay is more sensitive than serum protein electrophoresis and urine electrophoresis for detecting light chain only myeloma, AL amyloidosis and light chain deposition disease, and it tracks response in oligosecretory disease.

Monoclonal gammopathy of undetermined significance is present in about 3% of people over 50 years and 5% over 70 years, and an abnormal ratio is found in up to 1 in 10 of these. Multiple myeloma accounts for about 1% of cancers, and 15 to 20% are light chain only. AL amyloidosis affects 10 to 15 people in every million each year. Free light chains are cleared by the kidney, so levels rise with falling glomerular filtration and cast nephropathy is the main renal complication of myeloma.

Indications. Suspected myeloma, AL amyloidosis, light chain deposition disease, Waldenstrom macroglobulinaemia or POEMS syndrome. Unexplained renal failure, nephrotic syndrome, peripheral neuropathy, heart failure with restrictive features, hypercalcaemia, bone pain with anaemia, or raised total protein with low albumin. Monitoring of monoclonal gammopathy of undetermined significance, smouldering myeloma and treated myeloma (especially oligosecretory or non-secretory). Always requested together with serum protein electrophoresis and immunofixation.

Specimen. Serum 1 to 2 mL (gel tubes acceptable), no fasting. Stable for 7 days at 2 to 8 °C. The same assay manufacturer is used for serial results, because assays are not interchangeable.

Reference intervals (assay dependent, typical values).

• Free kappa 3.3 to 19.4 mg/L.
• Free lambda 5.7 to 26.3 mg/L.
• Kappa to lambda ratio 0.26 to 1.65.
• In renal impairment (eGFR under 60 mL/min/1.73 m²) the renal reference range for the ratio is 0.37 to 3.1, because clearance of lambda (monomer dimer difference) and kappa falls unequally.

Interpretation.

• Ratio over 1.65 (over 3.1 in renal impairment): clonal kappa excess.
• Ratio under 0.26 (under 0.37 in renal impairment): clonal lambda excess.
• Both chains raised with a normal ratio: polyclonal increase from renal failure, inflammation, infection or autoimmune disease.
• Both chains low with a low total: immunoparesis, seen in advanced myeloma or hypogammaglobulinaemia.

Diagnostic use.

• Myeloma: an involved to uninvolved ratio of 100 or more with an involved chain of 100 mg/L or more, in the presence of end organ damage, is a myeloma defining event in people with clonal plasma cells on marrow.
• Light chain myeloma: abnormal ratio with a normal or absent paraprotein on electrophoresis.
• AL amyloidosis: abnormal ratio in about 90% when combined with electrophoresis and immunofixation, rising to 99% with all three. The difference between involved and uninvolved chains (dFLC) is used for prognosis and response, with a dFLC over 180 mg/L as a high risk marker.
• Monoclonal gammopathy of undetermined significance: an abnormal ratio, a paraprotein under 30 g/L and under 10% marrow plasma cells place patients at higher risk of progression (about 1% a year overall, 2 to 3% a year with an abnormal ratio and non-IgG type).
• Smouldering myeloma: marrow plasma cells 10 to 60% or paraprotein 30 g/L or more, without end organ damage, with ratio 8 or more indicating higher risk.

Response assessment. Complete response requires a normal ratio with negative immunofixation and under 5% marrow plasma cells. Stringent complete response adds a normal ratio and absence of clonal cells. In patients with unmeasurable disease on electrophoresis, a drop of the involved chain by 50% or more denotes partial response.

Critical values and findings needing immediate action.

• Involved chain over 500 mg/L with acute kidney injury: probable cast nephropathy, a haematological emergency with urgent hydration and chemotherapy to reduce light chains.
• Ratio over 100 with end organ damage (renal failure, hypercalcaemia, anaemia, bone lesions): start myeloma workup and referral.
• Abnormal ratio with proteinuria, restrictive cardiomyopathy or neuropathy: urgent assessment for AL amyloidosis (tissue biopsy, troponin and NT-proBNP, echocardiography).
• Hyperviscosity symptoms (bleeding, visual change, confusion) with high paraprotein: urgent plasma exchange.

Interferences and pitfalls.

• Antigen excess: very high free light chain concentrations can give falsely low results on nephelometers. Automated dilution checks should be run, and the laboratory is told when clinical suspicion is high and the result is unexpectedly low.
• Renal impairment raises both chains and moves the ratio, so the renal reference range applies.
• Polyclonal increases (infection, inflammation, rheumatoid arthritis, lupus) give high chains with a normal ratio, and should not be reported as clonal.
• Different manufacturers' assays give different values, so serial monitoring uses one platform.
• Monoclonal immunoglobulin therapy such as daratumumab interferes with immunofixation, and sFLC becomes the main marker for response.
• Isolated light chain monoclonal proteins in the urine, with normal serum chains, can occur and prompt urine electrophoresis.
• The ratio is a clonality marker, not a tumour burden measure, in a patient with a stable monoclonal gammopathy.

Special groups.

• Older adults: clonal gammopathy of undetermined significance is frequent, and mild renal impairment widens the ratio.
• Dialysis patients: high flux membranes clear free chains and affect interpretation.
• Regions: myeloma is more frequent in people of African descent, with a two to three times higher incidence and earlier onset.

Follow up tests.

• Serum protein electrophoresis and immunofixation.
• 24-hour urine or spot urine protein electrophoresis and immunofixation.
• Quantitative immunoglobulins, full blood count and film.
• Calcium, creatinine and eGFR, albumin, beta-2 microglobulin and lactate dehydrogenase.
• Bone marrow aspirate and trephine with cytogenetics and FISH.
• Skeletal imaging by whole body low dose CT or MRI.
• In suspected amyloidosis: tissue biopsy with Congo red staining, troponin and NT-proBNP.