Free Kappa Light Chains, Serum
A small antibody piece (kappa type) made by plasma cells. It is read with lambda, the ratio and your kidney function.
AI-verified against cited sources. Awaiting expert reviewAlso called Kappa FLCLOINC 36916-5
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What it is
Antibodies are built from heavy chains and light chains. Light chains come in two types, kappa and lambda. Plasma cells make a little more light chain than they need, and the extra pieces float free in the blood. Your kidneys clear them, so the level depends on how much your plasma cells make and how well your kidneys work. This test measures the free kappa pieces. It is almost always ordered with free lambda, and the two are compared as a ratio.
Why it matters
Free light chain tests are used with protein electrophoresis and immunofixation to look into plasma cell conditions such as monoclonal gammopathy of undetermined significance (MGUS), multiple myeloma and light chain (AL) amyloidosis. They are also used to follow people who already have one of these conditions. A single kappa number says little on its own. Reduced kidney function raises both kappa and lambda, and so can long-term inflammation. That is why clinicians read kappa together with lambda, the kappa/lambda ratio and your kidney tests.
If it’s high
A kappa level above your lab's range is common and has many explanations. The most common is reduced kidney function, because the kidneys clear light chains from the blood. Long-term inflammation can also raise kappa and lambda together. When kappa is high and lambda is not, so that the ratio is also outside the range, your clinician may look for a plasma cell condition with more tests. Many other things can cause a high number, so your clinician can put it in context.
Common causes
- Reduced kidney function, which raises kappa and lambda together
- Older age, since kidney function tends to decline with age
- Long-term inflammation or autoimmune conditions, which can raise both chains
- Differences between lab methods, since each brand gives different numbers
- Natural variation that standard ranges do not capture, which studies have described in people of African ancestry
- Monoclonal gammopathy of undetermined significance (MGUS), including light chain MGUS
- Multiple myeloma, AL amyloidosis or related plasma cell or lymphoid conditions
What can help
- Ask your clinician how your kidney function (eGFR) fits with this result
- Ask whether the kappa/lambda ratio, protein electrophoresis and immunofixation were checked, since they are read together
- If you are followed over time, use the same lab and method each time, because methods are not interchangeable
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 3.3–19.4 mg/L. Source: Clinical Chemistry, ARUP Laboratories, University of Iowa Hospitals & Clinics Pathology Handbook, Mayo Clinic Laboratories (Extended Catalog), Biochemia Medica, Blood Cancer Journal.
- Adults (this one applies)3.3–19.4 mg/L
Sources: Clinical Chemistry, ARUP Laboratories, University of Iowa Hospitals & Clinics Pathology Handbook, Mayo Clinic Laboratories (Extended Catalog), Biochemia Medica, Blood Cancer Journal
What can affect your result
- Kidney function: lower eGFR raises both chains. Among about 6,500 people with eGFR below 60 and no sign of a monoclonal protein, 60% had kappa above 19.4 mg/L
- Age: levels rise with age, largely because kidney function declines
- Ancestry: standard ranges were built mostly from White participants, and a 2025 study of about 9,000 people proposed wider ranges for people of African ancestry
- Inflammation, autoimmune disease and high levels of normal antibodies (polyclonal hypergammaglobulinemia) can raise both chains
- Lab method: Freelite and N Latex FLC give different values and use different ranges
- Kidney function: reduced eGFR raises both chains, so a result outside the range does not by itself point to a plasma cell condition.
- Methods differ: Freelite and N Latex FLC give different values and use different ranges, and some labs print their own intervals. Results from different methods are not interchangeable.
- Antigen excess: very high amounts of one light chain can read falsely low on some assays. Labs run dilutions to catch this, but ARUP notes that it cannot be fully excluded.
- Sample type: ARUP accepts serum only and rejects plasma and grossly hemolyzed or lipemic samples. Some hospital labs, such as the University of Iowa, use EDTA plasma.
- Chronic inflammation or autoimmune disease can raise both chains together (a polyclonal increase).
Free light chains leave the blood within hours when the kidneys work normally (reported half-lives are roughly 2 to 6 hours), so levels can respond quickly to change. Clearance is slower in kidney disease, up to about 30 hours in advanced chronic kidney disease.
When to talk to a clinician
If your kappa/lambda ratio is outside your lab's range, mention it to your clinician, who can read it alongside your kidney function and other tests. Talk with your care team promptly if you have been told you have a plasma cell condition and your numbers are rising. If you already have a plasma cell condition, contact your care team promptly about new bone pain, new weakness or tiredness, signs of high calcium (such as thirst, constipation or confusion), or a drop in kidney function. These match the organ-damage features (calcium, kidney, anemia, bone) used to define myeloma.
Questions you could ask
- Which method does my lab use, and should I use the same lab for future tests?
- Why was this test ordered for me?
- Could my kidney function explain this result?
- Were the lambda result, the ratio, protein electrophoresis and immunofixation checked as well?
- Should we repeat this test, and when?
Questions people ask
Why is my kappa high when my lambda is high too?
When both chains are high and the ratio is inside the range, the most common reason is that your kidneys clear light chains more slowly. Long-term inflammation can do the same. This pattern is common in people with reduced kidney function. Your clinician will read the numbers together with your eGFR and the ratio.
Why does my lab report kappa in mg/dL when others use mg/L?
Both units are in use. One mg/dL equals 10 mg/L, so 1.94 mg/dL is the same as 19.4 mg/L. Always compare your result with the range printed on the same report.
Can I compare results from two different labs?
Not reliably. Freelite and N Latex give different values, and some labs print their own ranges. Comparisons over time work best when the same lab and method are used.
Show the science and sources
Units
- 1 mg/L = 1 mg/L
- 1 mg/L = 0.1 mg/dL
Open the Free kappa unit converter
For clinicians and researchers
Each plasma cell makes slightly more light chain than heavy chain, and the excess is released as free light chains (FLC). Kappa circulates mainly as a monomer and lambda mainly as a dimer. FLCs are filtered by the glomerulus and broken down in the proximal tubule. Reported half-lives are a few hours with normal kidney function (about 2–4 h in one review and 3–6 h in another) and can reach about 30 h in advanced CKD. Serum FLC is measured by immunoassay on nephelometers or turbidimeters. Freelite (The Binding Site, polyclonal antibodies) is the assay behind the IMWG guidelines and the intervals used here: kappa 3.3–19.4 mg/L, lambda 5.7–26.3 mg/L and a kappa/lambda ratio of 0.26–1.65, from adults aged 21 to 90 with normal kidney function (Katzmann 2002; about 280 people according to the iStopMM paper). N Latex FLC (Siemens, monoclonal antibodies) has manufacturer intervals of kappa 6.7–22.4 mg/L, lambda 8.3–27.0 mg/L and ratio 0.31–1.56. In a comparison of 93 samples the two assays correlated well but were not equivalent: Freelite read higher kappa and ratio values and lower lambda values, and 61 samples had a ratio outside the range on Freelite against 38 on N Latex. Mayo Clinic Laboratories prints its own intervals for its turbidimetric method (kappa 0.61–4.02 mg/dL, lambda 0.67–3.46 mg/dL, ratio 0.57–2.45). Antigen excess, where a very high monoclonal light chain reads falsely low, is rare, but ARUP notes it cannot be excluded. Because the kidneys clear FLCs, both chains rise as eGFR falls. In the Icelandic iStopMM cohort of about 6,500 people with eGFR below 60, no evidence of monoclonality and Freelite testing, the authors proposed central 99% intervals for kappa of 7.8–83.6 mg/L (eGFR 45–59), 8.8–103.3 mg/L (30–44) and 11.7–265.1 mg/L (below 30). These are research proposals and are not printed by labs. A 2025 study of about 9,000 people (Blood) found that standard ranges flagged light chain MGUS in 14.8% of Black and 4% of White US participants, and proposed ranges of 7.97–77.50 mg/L for kappa and 6.20–49.20 mg/L for lambda in people of African ancestry with normal kidney function. The IMWG (2009) recommends the serum FLC assay with serum protein electrophoresis and immunofixation for screening, which removes the need for 24-hour urine studies for diagnoses other than AL amyloidosis. It calls the baseline FLC prognostic in nearly every plasma cell disorder and supports serial FLC for oligosecretory disease, but found no data to support replacing 24-hour urine electrophoresis when disease is measurable in serum or urine. In the 2014 IMWG criteria, an involved-to-uninvolved FLC ratio of 100 or more with an involved FLC of 100 mg/L or more is a myeloma-defining event. Light chain MGUS is defined as a ratio outside the reference interval with a raised involved chain, no heavy chain on electrophoresis or immunofixation, and no organ damage attributed to the plasma cell disorder.
LOINC 36916-5 (primary), 104544-2, 80515-0. Specimen: serum.
Sources
- Serum reference intervals and diagnostic ranges for free kappa and free lambda immunoglobulin light chains: relative sensitivity for detection of monoclonal light chains, Clinical Chemistry (2002)
- Kappa/Lambda Quantitative Free Light Chains with Ratio, Serum (test 0055167), ARUP Laboratories
- Kappa-Lambda Quant Free Light Chain Ratio, Blood, University of Iowa Hospitals & Clinics Pathology Handbook
- Immunoglobulin Free Light Chains, Serum (test FLCS), Mayo Clinic Laboratories (Extended Catalog)
- Comparison of Freelite and N-Latex serum free light chain assays: a critical review, Biochemia Medica (2021)
- Defining new reference intervals for serum free light chains in individuals with chronic kidney disease: Results of the iStopMM study, Blood Cancer Journal (2022)
- International Myeloma Working Group guidelines for serum-free light chain analysis in multiple myeloma and related disorders, Leukemia (2009)
- International Myeloma Working Group updated criteria for the diagnosis of multiple myeloma, The Lancet Oncology (2014)
- Serum free light chains in a racially diverse population including African Americans and populations from South Africa, Blood (2025)
- Diagnostic and Therapeutic Aspects of Monoclonal Gammopathies of Renal Significance (MGRS): An Update, Diagnostics (Basel) (2024)
- Extracorporeal Free-Light-Chain Removal in Myeloma Cast Nephropathy: Plasma Exchange and Hemodialysis Strategies in the Era of Effective Antimyeloma Therapy, Journal of Clinical Medicine (2026)
- Elevated serum polyclonal immunoglobulin free light chains in patients with severe asthma, Frontiers in Pharmacology (2023)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.