Practical NeurologyKappa Free Light Chains in the 2024 McDonald Criteria: What Neurologists Need to Know

✅ Guideline Update

The revised McDonald criteria expand the laboratory assessment of suspected multiple sclerosis by recognizing the κFLC index as an alternative to CSF-restricted oligoclonal bands for establishing positive CSF. Unlike standalone CSF κFLC concentration, the index incorporates paired CSF and serum κFLC plus albumin measurements to account for blood-brain barrier permeability. The change introduces a quantitative, potentially automated approach to detecting intrathecal immunoglobulin synthesis, but κFLC remains one component of an integrated diagnosis rather than a standalone MS test. Internal MS landscape materials likewise identify κFLC as an alternative to OCB testing under the revised criteria.

Clinical Considerations

• A κFLC index ≥6.1 is described in the source as defining positive CSF when appropriately validated assays are used.
• Calculating the index requires paired CSF and serum κFLC plus paired CSF and serum albumin, distinguishing it from isolated CSF κFLC concentration.
• The article reports approximately 90% concordance with OCB testing, supporting selected use of the alternate method when results are discordant or difficult to interpret.
• Assays are not analytically interchangeable. Internal assay-validation documentation likewise cautions that results obtained with different κFLC assays or systems cannot be used interchangeably.
• A positive κFLC index does not independently establish MS; evidence of intrathecal immunoglobulin synthesis can occur in other inflammatory CNS disorders.
• OCB testing remains clinically relevant, particularly when κFLC findings conflict with a strongly suggestive clinical and MRI presentation.

Practice Applications

• Recognize the κFLC index as an accepted alternative approach to demonstrating positive CSF within the revised MS diagnostic criteria.
• Differentiate the κFLC index from standalone CSF κFLC concentration when reviewing laboratory results.
• Integrate κFLC findings with clinical presentation, MRI findings, and assessment for alternative diagnoses rather than interpreting the biomarker independently.
• Review the laboratory’s assay, validation, decision threshold, and interpretive information when incorporating κFLC results into diagnostic assessment.
• Consider OCB testing in clinically appropriate discordant or equivocal situations rather than regarding it as obsolete.