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Serum free light chain (FLC) testing plays an important role in the diagnosis and monitoring of plasma cell disorders, including multiple myeloma. However, many laboratories continue to use κ/λ ratio reference intervals established more than two decades ago from relatively small and limited populations. Even current International Myeloma Working Group (IMWG) recommendations continue to rely on these historical reference intervals1. As patient demographics, analytical platforms, and assay methodologies evolve, increasing concerns have emerged regarding whether these older reference intervals remain clinically appropriate
One major challenge is the substantial variability observed between commercially available FLC assays, including Freelite, Diazyme, Siemens N Latex, and Sebia. Differences in antibody design, assay calibration, and analytical methodology can lead to significantly different numerical κ/λ ratio results between platforms2. Importantly, no international reference measurement system or fully harmonized calibration currently exists for serum FLC assays, further contributing to inter-assay variability. Additional concerns have also been raised regarding Freelite assay drift over time3 and the strong influence of kidney function4 on serum FLC concentrations, both of which may further affect clinical interpretation and reference interval suitability.
These analytical differences have important clinical implications. A CAP proficiency testing study by Susan L. Fink5 demonstrated that the same patient sample with an FLC ratio near the upper reference limit produced widely different results across platforms, ranging from 1.09 to 3.70. Consequently, 40.3% of laboratories interpreted the sample as “Normal,” while 59.7% classified it as “Abnormal, High.” Despite this variability, most laboratories continued using the historical 0.26–1.65 κ/λ ratio reference interval, highlighting the potential risk of inconsistent patient classification when uniform cutoffs are applied across different assays.
The concern becomes even more significant when biological factors such as age and kidney function are considered. The recent iStopMM study6 challenged the use of universal FLC reference intervals. In more than 75,000 Icelandic participants, the study showed that age and kidney function significantly affect serum FLC concentrations and κ/λ ratios. Biologically adjusted reference intervals based on age and eGFR reduced potential false positive abnormal classifications, particularly in older adults and patients with chronic kidney disease. However, because the study population was predominantly Icelandic, the findings may not fully apply to more ethnically diverse populations. More recently, validation studies using the revised iStopMM reference intervals demonstrated improved MGUS risk stratification and reduced overdiagnosis. In a Danish MGUS cohort7, 32% of individuals with abnormal FLC ratios by historical intervals were reclassified as normal using the revised intervals without increased progression risk. These findings further support the clinical value of biologically adjusted FLC reference intervals.
Together, these studies highlight the growing need for assay-specific, population-appropriate, and biologically adjusted FLC reference intervals. Re-evaluation of current κ/λ ratio cutoffs may improve harmonization between assays, reduce false positive interpretations, and enhance the clinical utility of serum FLC testing.
References
- Rajkumar SV, Dimopoulos MA, Palumbo A, et al. International Myeloma Working Group updated criteria for the diagnosis of multiple myeloma. The Lancet Oncology. November 2014. Available from: The Lancet Oncology
- Giles HV, Karunanithi K. Performance Characteristics and Limitations of the Available Assays for the Detection and Quantitation of Monoclonal Free Light Chains and New Emerging Methodologies. Antibodies. March 11, 2024. Available from: Antibodies
- Murray D, Dispenzieri A, Kumar S, et al. Free Light Chain Assay Drift: Potential for Misdiagnosis? The Journal of Applied Laboratory Medicine. August 23, 2020. Available from: The Journal of Applied Laboratory Medicine
- Yadav P, Cockwell P, Cook M, et al. Serum free light chain levels and renal function at diagnosis in patients with multiple myeloma. BMC Nephrology. July 13, 2018. Available from: BMC Nephrology
- Fink SL, Wener MH, Rudolf JW, et al. A universal reference interval for serum immunoglobulins free light chains may be outdated. Clinical Chemistry and Laboratory Medicine. May 25, 2023. Available from: Clinical Chemistry and Laboratory Medicine
- Rognvaldsson S, Love TJ, Thorsteinsdottir S, et al. Iceland screens, treats, or prevents multiple myeloma (iStopMM): a population-based screening study for monoclonal gammopathy of undetermined significance and randomized controlled trial of follow-up strategies. Blood Cancer Journal. May 17, 2021. Available from: Blood Cancer Journal
- Maeng CV, Rognvaldsson S, Long TE, et al. Revised free light chain reference intervals enhance risk stratification in monoclonal gammopathy of undetermined significance and reduce overdiagnosis. Blood Cancer Journal. April 28, 2025. Available from: Blood Cancer Journal
