Kappa Free Light Chain Unit Converter
Kappa Free Light Chain Unit Converter
mg/L, mg/dL and µg/mL for free kappa light chains — and the point the units conceal, which is that a Freelite result and an N Latex result are different numbers for the same serum and cannot be compared.
Kappa Free Light Chain converter
mg/L ⇄ mg/dLFree kappa light chains 12.5 mg/L, measured on Freelite
Units and conversion
mg/dL = mg/L ÷ 10
mg/L = mg/dL × 10
- mg/L and µg/mL
- the same number. One milligram per litre is one microgram per millilitre, because the thousandfold in the mass cancels the thousandfold in the volume
- mg/dL
- a tenth of the mg/L figure. Reporting free light chains in mg/dL is uncommon outside a few laboratories, and a result read in the wrong one is out by a factor of ten
- no molar unit
- free kappa light chains circulate as a mixture of monomers and dimers, and the assay measures an epitope exposed only when the chain is unbound rather than a defined molecular species. There is no single mass to divide by, and the result is a calibrator-defined mass concentration rather than a stoichiometric one
- calibrator dependence
- the reason the assay is a selectable option above rather than a footnote. Freelite uses polyclonal antisera, N Latex FLC monoclonal antibodies, and the two give systematically different numbers on the same serum. Their reference intervals differ accordingly, and the results are not interchangeable
- IMWG
- the International Myeloma Working Group criteria — including the involved:uninvolved ratio of 100 or more with the involved chain at 100 mg/L or more — were derived on Freelite values. Applying them to an N Latex result is an extrapolation, not a reading
Worked example
Free kappa light chains 12.5 mg/L, measured on Freelite
12.5 mg/L = 12.50 µg/mL — identical units
12.5 ÷ 10 = 1.250 mg/dL
Within the Freelite interval of 3.3–19.4 mg/L
On N Latex FLC the interval would be 6.7–22.4 mg/L, and the measured number itself would differ — the two assays are not interchangeable
Reference intervals by assay
| Assay | Free kappa (mg/L) | Free lambda (mg/L) | κ:λ ratio |
|---|---|---|---|
| Freelite (The Binding Site) | 3.3 – 19.4 | 5.7 – 26.3 | 0.26 – 1.65 |
| N Latex FLC (Siemens) | 6.7 – 22.4 | 8.3 – 27.0 | 0.31 – 1.56 |
Why one chain is raised
| Cause | What happens to both chains | What happens to the ratio |
|---|---|---|
| Monoclonal plasma cell clone | One chain rises, the other is normal or suppressed | Abnormal — this is the finding the test exists for |
| Reduced glomerular filtration | Both rise together, since both are cleared renally | Broadly preserved, but drifts toward kappa; the accepted range widens to about 0.37–3.10 |
| Polyclonal immune stimulation — infection, inflammation, autoimmune disease | Both rise together | Normal |
| Immunoparesis in advanced myeloma | The uninvolved chain is suppressed | Exaggerated, because the denominator falls as well as the numerator rising |
Free kappa light chains: the units are easy, the assay is not
Plasma cells make slightly more light chain than they need to assemble immunoglobulin, and the surplus circulates unbound. Free kappa light chains are cleared almost entirely by glomerular filtration and proximal tubular catabolism, with a serum half-life measured in hours rather than the weeks of an intact immunoglobulin, which is what makes the measurement responsive enough to track a clone. They are reported as a mass concentration in mg/L, which is numerically identical to µg/mL; mg/dL, used by a few laboratories, is the mg/L figure divided by ten.
There is no molar unit, and the reason is worth stating plainly. The assay does not measure a defined molecular species: it detects an epitope on the light chain that is buried when the chain is bound into an immunoglobulin and exposed when it is free, and the free chains themselves circulate as a mixture of monomers and dimers. What comes out is a concentration defined by the calibrator, not by a molecular weight, and that has a consequence far more important than the missing nmol/L column.
Two assays dominate, and they do not agree. Freelite uses polyclonal antisera raised against hidden light chain epitopes; N Latex FLC uses monoclonal antibodies. Run on the same serum they return systematically different numbers, and their reference intervals differ accordingly — for free kappa, 3.3 to 19.4 mg/L on Freelite against 6.7 to 22.4 mg/L on N Latex. A comparative review concluded the two should not be considered equivalent and are not interchangeable, and found patients whose ratio was abnormal on one assay and normal on the other. Practically: a patient must be monitored on one platform throughout, a result must never be trended against a result from another laboratory using the other assay, and the International Myeloma Working Group thresholds — including the involved:uninvolved ratio of 100 with the involved chain at 100 mg/L — were derived on Freelite and are an extrapolation anywhere else.
Renal function is the other thing to settle before interpreting a raised value. Both chains are cleared by the kidney, so a fall in glomerular filtration raises both together; in a patient with chronic kidney disease a raised free kappa says more about the kidney than about the marrow. The ratio is what separates that from a clone, and even the ratio needs the renal correction — the accepted range widens from 0.26–1.65 to roughly 0.37–3.10 in renal impairment. Neither this page nor any single absolute concentration can make that call; calculate the ratio and read it against the eGFR.
Frequently asked questions
How do I convert free kappa light chains from mg/L to mg/dL?
Divide by ten: 12 mg/L is 1.2 mg/dL. µg/mL is numerically identical to mg/L, so no conversion is needed between those two.
Can I compare a Freelite result with an N Latex result?
No. The two assays use different antibodies and different calibrators and return systematically different concentrations on the same serum, with different reference intervals to match. A published comparison concluded they are not interchangeable. Monitor a patient on one platform throughout.
Why is there no nmol/L for free kappa light chains?
The assay detects an epitope exposed only when the chain is unbound, and free chains circulate as both monomers and dimers, so there is no single molecular species with a defined mass to convert through. The result is a calibrator-defined mass concentration.
Does a raised free kappa light chain mean myeloma?
Not on its own. Reduced glomerular filtration raises both chains together, as does polyclonal immune stimulation from infection or inflammation. What points to a clone is an abnormal ratio between the two chains, read alongside renal function and serum protein electrophoresis.
What is a normal free kappa light chain level?
3.3 to 19.4 mg/L on Freelite and 6.7 to 22.4 mg/L on N Latex FLC, both assuming normal renal function. The two are not the same measurement, so the interval has to match the assay printed on the report.
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References
- Katzmann JA, Clark RJ, Abraham RS, et al. Serum reference intervals and diagnostic ranges for free kappa and free lambda immunoglobulin light chains: relative sensitivity for detection of monoclonal light chains. Clin Chem. 2002;48(9):1437–1444.
- Daves M, Piccin A, Roccaforte V, Lippi G. Comparison of Freelite and N-Latex serum free light chain assays: a critical review. Biochem Med (Zagreb). 2021;31(3):030701.
- Rajkumar SV, Dimopoulos MA, Palumbo A, et al. International Myeloma Working Group updated criteria for the diagnosis of multiple myeloma. Lancet Oncol. 2014;15(12):e538–e548.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.