Diagnostic accuracy of salivary and serum-free light chain assays in primary Sjogren's syndrome: a pilot study.

OBJECTIVE: To estimate levels of salivary and serum free light chains (FLCs) and explore its utility as a biomarker in primary Sjogren's syndrome (pSS). METHODS: Patients with pSS classified by American European Consensus group 2002 or American College of Rheumatology 2012 criteria between January 2015 and August 2015 were included. Healthy staff and non-first degree relatives of patients constituted controls. Serum and salivary FLCs were measured by immunoturbidometry using FREELITE() Human Kappa(kappa) and Lambda(lambda) Free Kit (Binding site, Birmingham, UK), on a Roche Modular P800. FLCs were compared between cases and controls using the Mann-Whitney U-test. The receiver operator characteristic curve was constructed to analyze the discriminating ability of salivary and serum kappa and lambda FLCs. RESULTS: Salivary and serum FLCs were assayed in 15 patients and 13 patients, respectively, and in 15 controls. Median age of cases and controls was 34 years. Salivary kappa and lambda FLCs were higher in pSS as compared to controls (P < 0.05 and P < 0.001, respectively). Serum kappa and lambda FLCs were also higher in pSS (both P < 0.05). Salivary lambda levels were higher in pSS with ocular signs; serum kappa and lambda levels were higher in those with ocular symptoms. A cut off of >/= 1.1 mg/L for salivary lambda FLC had a sensitivity and specificity of 73.3% and 93.3%, respectively, for the diagnosis of pSS. Serum kappa FLC >/= 30 mg/L had a sensitivity and specificity of 92.3% and 73.3%, respectively. CONCLUSION: Serum and salivary FLCs and in particular the latter, are potential biomarkers in pSS. Larger studies are required for validating the findings.