Vincent Rajkumar: A quick primer on interpreting the serum FLC ratio

Vincent Rajkumar shared a post on X:

“A quick primer on how to interpret the serum FLC ratio or level.”

Quoting his earlier post:

“Recs for using free light chains (FLC) in practice:

We get a lot of consults for abnormal FLC ratios/levels. If FLCs are the only abnormality, you don’t need to do much unless involved/uninvolved ratio is >8.

Don’t chase minor abnormalities.

If there is clinical concern for light chain process like amyloid or if there is intact immunoglobulin M spike, u can do more. But uncomplicated FLC ratio <8 is almost never going to be high risk SMM.

Light chain MGUS with FLC ratio <8 is managed like low risk MGUS.

When we first established the normal FLC range it was a new test and the consequences of abnormal value was not known. A very strict cut off was used. We know now that the normal ratio is affected by age and creatinine.

An abnormal FLC ratio >20 in a patient with smoldering myeloma indicates high risk state if either bone marrow plasma cells >20 or M spike >2. Mayo 20-2-20 criteria.

You can fine tune using this scoring system.

>=10% clonal marrow plasma cells + FLC ratio >100 is used as a myeloma defining event.

But in some, FLC ratio can be markedly elevated due to dimerization of light chains, not excess production. Check urine M protein; if urine levels are very low it’s not myeloma.”

How I manage monoclonal gammopathy of undetermined significance

Authors: Ronald S. Go, S. Vincent Rajkumar.

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Vincent Rajkumar is a Professor of Medicine at the Mayo Clinic in Rochester, Minnesota, and Chair for the Mayo Clinic Myeloma, Amyloidosis, and Dysproteinemia Group.  He also chairs the Board of directors of  The International Myeloma Foundation and the Eastern Cooperative Oncology Group (ECOG) Myeloma Committee. His extensive contributions include over 230 peer-reviewed publications, predominantly focusing on multiple myeloma and related plasma cell disorders. Furthermore, Dr. Rajkumar is a Section Editor for multiple myeloma and related disorders for Leukemia and an Associate Editor for the Mayo Clinic Proceedings.