Proteomic profiling of serum and saliva in rheumatoid arthritis: A pilot Q-TOF study.

Background: Rheumatoid arthritis (RA) is a chronic autoimmune disease that primarily targets joints but also affects multiple organ systems. Current RA diagnostics rely on serum biomarkers like rheumatoid factor and anti-citrullinated protein antibodies, which indicate systemic inflammation but often lack sensitivity in the early stages. Recent advances in proteomics provide opportunities for discovering new biomarkers that could enhance diagnostic accuracy. Saliva, a non-invasive biofluid, has emerged as a potential alternative that may reflect both systemic and localized inflammation. Objective: This pilot study assesses whether saliva can complement serum in diagnosing and monitoring RA. Using quadrupole time-of-flight mass spectrometry, we compare serum and saliva proteomic profiles to identify RA-related biomarkers. Materials and Methods: Serum and saliva samples from five RA patients and five healthy controls were analyzed using quadrupole time-of-flight mass spectrometry. Multivariate analyses, including partial least squares discriminant analysis and receiver operating characteristic analysis, were used to evaluate the diagnostic potential of saliva-based biomarkers relative to those in serum. Results: RA-related biomarkers, including citrullinated vimentin and tyrosine, were identified in both serum and saliva. Partial least squares discriminant analysis demonstrated distinct separation among sample groups, while receiver operating characteristic analysis indicated high diagnostic accuracy, supporting saliva's utility as a non-invasive complement to serum. Conclusion: Saliva shows promise as a complementary biofluid for RA diagnostics. It allows non-invasive sampling and the potential for frequent monitoring. While serum remains the standard for diagnosis, saliva could enhance disease monitoring in clinical practice.