Detection of light chain (LC) monoclonal gammopathies (MG) traditionally relies on serum free LC (FLC) κ, λ, and their ratio (κ/λ) reference ranges based on a mostly White population. We investigated FLC values in a racially diverse population by screening 10,035 individuals for heavy chain MG, identifying 9,028 negative cases whose FLC were measured. Participants included 4,149 from the PROMISE Study (US, n=2,383; South Africa, n=1,766) and 4,879 from the Mass General Brigham Biobank, with …