Kappa free light chain index or oligoclonal bands testing in clinical practice for multiple sclerosis diagnosis: An algorithm comparative study

BACKGROUND: Limited evidence exists on how to use kappa free light chains (κFLCs) and oligoclonal bands (OCBs) in diagnosing intrathecal immunoglobulin synthesis (IgS). OBJECTIVE: To compare different algorithms to define IgS. METHODS: A retrospective analysis of the 2022 French cohort study was conducted. Patients were considered to have IgS if (1) the κFLC index was ⩾6.1, (2) OCB was positive, (3) the κFLC index was ⩾6.1 or OCB was positive, (4) the κFLC index was ⩾20 or OCB was positive, regardless of the κFLC index value, or (5) the κFLC index was ⩾20 or OCB was positive (with OCB tested only when the κFLC index was between 3.5 and 20). People with multiple sclerosis (MS) were compared with controls and with a subgroup of age- and sex-matched MS-mimicking disorders. RESULTS: Of 1531 patients, 675 had MS and 856 were controls (133 MS mimics). Agreement between OCB and κFLC index (⩾6.1) was high (89%). All tested algorithms demonstrated high accuracy in separating groups. Using the sequential approach (5) achieves higher specificity and a significant reduction in OCB testing. CONCLUSION: Using a simple cut-off point of 6.1 for routine screening is accurate, but testing for OCB when the κFLC index is between 3.5 and 20 increases specificity and should be prospectively studied in challenging cases.