Validation and optimization of reference intervals for serum free light chains in Chinese patients with chronic...

To explore the applicability of the laboratory-specific sFLC reference intervals in patients with CKD and to define new sFLC reference intervals on N-Latex platform in Chinese CKD patients.

We retrospectively analyzed clinical data from 1,700 patients at all stages of CKD who attended Zhongshan Hospital, Fudan University, between 1 January 2023 and 31 December 2025; the cohort comprised 1,066 males and 538 females with 60 (IQR49-69) years. sFLC concentrations and κ/λ ratio were characterized, and stage-specific 95% reference intervals were derived from 1,135 randomly selected CKD patients and validated in other 565 patients. The clinical utility of these intervals was further validated with 36 patients initially diagnosed with LC-monoclonal gammopathy and eGFR < 60 mL/min/1.73 m².

The laboratory-specific reference intervals (κFLC 9.6-23.2 mg/L, λFLC 11.6-37.5 mg/L, κ/λ ratio 0.540-1.150) showed limitations in CKD patients, with 96.2%, 85.0% and 12.2% of individuals falling outside these intervals, respectively. We derived CKD-stage-specific 95% reference intervals stratified by eGFR: eGFR 30-59 mL/min/1.73 m²: κFLC 17.9-103.0 mg/L, λFLC 20.7-112.3 mg/L, κ/λ ratio 0.489-1.401; eGFR < 30 mL/min/1.73 m²: κFLC 42.5-297.8 mg/L, λFLC 49.7-338.8 mg/L, κ/λ ratio 0.518-1.245. The novel CKD-specific reference intervals led a reduction of 11.3% and 3.9% in abnormal κ/λ ratio rates among patients with eGFR 30-59 mL/min/1.73 m² and eGFR < 30 mL/min/1.73 m², respectively. All 36 LC-monoclonal gammopathy patients with eGFR < 60 mL/min/1.73 m² exhibited an abnormal κ/λ ratio ;none fell within the novel κ/λ ratio reference interval.

The CKD-specific sFLC reference intervals on N-Latex platform significantly reduce the proportion of abnormal sFLC results and enhance the application value of sFLC in CKD patients.