Effect of long-term frozen storage on stability of kappa free light chain index.

OBJECTIVES

To investigate whether frozen storage duration influences κ-FLC index.

METHODS

CSF and serum samples of patients with multiple sclerosis collected for routine diagnostic purposes had been stored at -20 °C. κ-FLC and albumin concentrations were measured at two different timepoints, i.e. before and after storage. The κ-FLC index was calculated as (CSF κ-FLC/serum κ-FLC)/(CSF albumin/serum albumin).

RESULTS

A total of 70 patients were included showing median CSF κ-FLC concentration of 0.25 (25th-75th percentile: 0.12-0.43) mg/dL, serum κ-FLC concentration of 1.05 (0.86-1.34) mg/dL, CSF albumin concentration of 17.7 (14.6-26.1) mg/dL and serum albumin concentration of 4230 (3898-4488) mg/dL. The κ-FLC index was 44 (25-108). With increasing frozen storage duration, the absolute concentrations of CSF κ-FLC, serum κ-FLC, CSF albumin and serum albumin decreased, while the κ-FLC index remained stable. The observed changes in absolute concentrations evened out by using CSF/serum ratios of κ-FLC and albumin when calculating the κ-FLC index.

CONCLUSIONS

Frozen storage at -20 °C has no relevant impact on κ-FLC index.