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External validation of KFLC index threshold for multiple sclerosis diagnosis in a French cohort
C Aguesse1,2, E Micoud3,4, S Vukusic1,2
1Service de Neurologie, Sclérose en Plaques, Pathologies de La Myéline Et Neuro-Inflammation, Hôpital Neurologique Pierre Wertheimer, Hospices Civils de Lyon, 69677, Bron, France.
Background:
The kappa free light chains (KFLC) index was recently incorporated into the 2024 McDonald criteria as a practical alternative to oligoclonal bands (OCB) in multiple sclerosis (MS) diagnosis. Our objectives were to compare the diagnostic performance of KFLC index and OCB, evaluate the recommended KFLC index threshold of 6.1, and assess the correlation between KFLC index and disease activity.
Methods:
We conducted a retrospective study on patients referred to the Lyon MS center who underwent lumbar puncture for OCB assessment from October 2019 to November 2023. KFLC index was measured on stored CSF and serum samples. Patients were classified as MS, other inflammatory neurological disease (OIND), or non-inflammatory neurological disease controls (NINDC).
Results:
We included 262 patients (153 MS, 62 OIND, 47 NINDC). The median KFLC index was significantly higher in MS patients (42 [IQR 13.5-108.3]) compared to OIND (2.7 [1.5-7.4], p < 0.001) and NINDC (1.6 [1-6.5], p < 0.001). The optimal threshold in our cohort was 5.48, yielding sensitivity of 89.5% and specificity of 83.5%, comparable to OCB (92.8% and 80.7%). The 6.1 threshold also performed well (sensitivity 87.6%, specificity 83.5%), though it may be suboptimal in male patients. KFLC index was significantly higher in active MS and in patients with high lesion load at imaging.
Conclusion:
Our study confirms the strong diagnostic performance of the KFLC index, suggests a correlation with disease activity and lesion load and supports the 6.1 threshold from the 2024 McDonald criteria, while highlighting the importance of maintaining OCB testing in selected cases.
