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The following is a summary of “Diagnostic Utility of Kappa Free Light Chain Index in Adults With Inaugural Optic Neuritis,” published in the March 2025 issue of Neurology Neuroimmunology & Neuroinflammation by Demortiere et al. 

Researchers conducted a retrospective study to evaluate a quick and reliable method for distinguishing multiple sclerosis (MS), myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD), and neuromyelitis optica spectrum disorder (NMOSD) in inaugural optic neuritis (ION). 

They included all adults admitted to the MS center of Marseille for ION between March 2016 and April 2024, with CSF analysis of the kappa free light chain (K-FLC) index. They used receiver operating characteristic curves to assess its diagnostic ability. 

The results showed that 227 adults were admitted for ION, and 210 (93%) had a K-FLC index measurement. MS was diagnosed in 84 (40%), CIS suggestive of MS in 77 (36.5%), including 20 with future conversion to MS (CISwc), MOGAD in 26 (12.5%), NMOSD in 13 (6%), and other inflammatory disorders in 10 (5%). A K-FLC index ≥6.7 differentiated MS/CISwc from other diagnoses with 86% specificity and 95% sensitivity (AUC 0.94). A K-FLC index <4.9 differentiated MOGAD from other diagnoses with 63% specificity and 92% sensitivity (AUC 0.78) and from MS/CISwc with 96% specificity and 92% sensitivity (AUC 0.97). Among 93 patients (44%) with a K-FLC index <4.9, 24 (26%) had MOGAD and 5 (5.5%) MS/CISwc. Among 117 patients with a K-FLC index ≥4.9, 2 (1.7%) had MOGAD (K-FLC index 7.9 and 16.2) and 99 (85%) MS/CISwc. Among 96 patients with normal MRI, 73 (76%) had a K-FLC index <4.9, with 22 (30%) having MOGAD and none converting to MS. Among 23 patients with a K-FLC index ≥4.9, 2 (8.5%) had MOGAD and 7 (30.5%) converted to MS. The K-FLC index did not differentiate NMOSD from other diagnoses and moderately differentiated NMOSD from MS/CISwc (AUC 0.80). 

Investigators found the K-FLC index to be an accessible biomarker for early diagnosis in patients with ION. They showed that the probability of MOGAD was low in patients with a K-FLC index ≥4.9, even with a normal brain or spinal cord MRI. 

Source: neurology.org/doi/10.1212/NXI.0000000000200386