Abstract
Background and aim: The International Myeloma Group Uniform response criteria for multiple myeloma mandates the normalisation of serum free light chain ratio (SFLCR) along with absent clonal plasmacytosis to report stringent complete treatment response(sCR).Inclusion of this requirement in addition to the criteria for clinical response(CR) implies that SFLCR is a more sensitive indicator of residual disease than serum immunofixation electrophoresis(SIFE). This study aimed to analyse the relationship between SFLCR and SIFE in patients of monoclonal gammopathies.
Methodology: This retrospective cross-sectional study was conducted in a referral laboratory of India. Records of 716 cases of confirmed monoclonal gammopathies including results of serum protein electrophoresis(SPE), SIFE and SFLC assay, at various stages of treatment ,were analysed. Reference range for SFLC ratio(κ:λ) was 0.26-1.65. The results of SFLC assay were categorised as concordant or discordant with established diagnosis based on correct or incorrect identification, respectively, of dominant light chain by the κ:λ ratio.
Results: Percentage of samples with normal SFLCR in presence of truly positive SIFE (21.5%) was substantially more than those with concordant abnormal SFLCR and negative SIFE(4.88%). The false negative rate for κ:λ ratio in detection of lambda chain lesions (55%) was higher than that for kappa light chain lesions(20%).
Conclusion: Normal SFLCR cannot exclude the presence of residual disease in monoclonal gammopathies. Results should be interpreted with caution while assessing response to treatment in such cases.

Key words: Serum free light chain assay, Serum protein electrophoresis, Serum immunofixation electrophoresis, Monoclonal gammopathy