Serum Free Light Chain Ratio Calculator
Serum Free Light Chain Ratio Calculator
Calculate the kappa:lambda free light chain ratio, and see why the normal range shifts in renal impairment.
Serum Free Light Chain Ratio
κ ÷ λFree kappa 42 mg/L, free lambda 12 mg/L
Formula
- normal range
- 0.26 – 1.65, in a person with normal renal function
- renal impairment
- widens to approximately 0.37 – 3.10, because both chains are cleared renally and rise together
- IMWG threshold
- an involved:uninvolved FLC ratio ≥100, with the involved FLC ≥100 mg/L, is one of the SLiM-CRAB biomarkers defining myeloma requiring treatment
Worked example
Free kappa 42 mg/L, free lambda 12 mg/L
42 ÷ 12 = 3.50
Above 3 → marked kappa excess, abnormal even allowing for renal impairment
Reference ranges and the IMWG threshold
| Setting | Normal κ:λ range | Note |
|---|---|---|
| Normal renal function | 0.26 – 1.65 | Standard laboratory reference interval |
| Renal impairment | 0.37 – 3.10 | Both chains rise with reduced clearance — using the standard range over-diagnoses monoclonal disease |
| Myeloma requiring treatment (SLiM-CRAB) | Involved:uninvolved ≥ 100 (involved FLC ≥ 100 mg/L) | One of three biomarkers defining progression from smouldering to active myeloma |
Reading the ratio, and why renal function matters
The free light chain ratio compares two proteins that are normally produced by polyclonal plasma cells in a fixed proportion and cleared almost entirely by the kidney. A monoclonal plasma cell population breaks that proportion, pushing the ratio toward whichever chain it secretes, so an abnormal ratio is a sensitive marker of clonality even when total protein electrophoresis looks unremarkable.
The single most important source of error is applying the standard 0.26–1.65 range to a patient with reduced renal function. Because both chains are cleared by the kidney, impairment raises both together and the ratio drifts toward kappa; the accepted range widens to roughly 0.37–3.10. Using the standard range in chronic kidney disease is the commonest cause of over-calling monoclonal disease from this test.
The test earns its place in light-chain-only and non-secretory myeloma and in AL amyloidosis, where serum and urine electrophoresis can be entirely normal despite a clonal process. An abnormal involved:uninvolved ratio of 100 or more, with the involved FLC at least 100 mg/L, is also one of the three IMWG biomarkers that redefine smouldering myeloma as myeloma requiring treatment. Polyclonal rises from inflammation or infection raise both chains together and, unlike renal impairment, usually leave the ratio itself normal.
Frequently asked questions
What is a normal free light chain ratio?
0.26 to 1.65 in a person with normal renal function. The range widens to about 0.37 to 3.10 in renal impairment, because both chains are cleared by the kidney and rise together.
Why does chronic kidney disease change the normal range?
Both free kappa and free lambda are cleared renally. As clearance falls, both rise, and the ratio drifts toward kappa even with no clonal process — applying the standard range then over-diagnoses monoclonal disease.
When is the free light chain ratio more useful than electrophoresis?
In light-chain-only myeloma, non-secretory myeloma, and AL amyloidosis, where serum protein electrophoresis can be normal despite significant clonal disease.
How does the free light chain ratio relate to myeloma diagnosis?
An abnormal involved:uninvolved ratio of 100 or higher, with the involved free light chain at least 100 mg/L, is one of the IMWG SLiM-CRAB criteria that define myeloma requiring treatment rather than smouldering disease.
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References
- Rajkumar SV et al. International Myeloma Working Group updated criteria for the diagnosis of multiple myeloma. Lancet Oncol. 2014;15(12):e538–48.
- Hutchison CA et al. Quantitative assessment of serum and urinary polyclonal free light chains in patients with chronic kidney disease. Clin J Am Soc Nephrol. 2008;3(6):1684–90.