Sjögren’s disease is a chronic systemic autoimmune disorder with a broad clinical spectrum, most commonly characterized by exocrine dysfunction leading to dry mouth and dry eyes. Other mucosal surfaces and the skin may also be affected. Extraglandular manifestations are also common. The disease is characterized by persistent systemic inflammation and impaired redox homeostasis; therefore, interest is increasing in the role of oxidative stress in its pathogenesis and in explaining clinical heterogeneity. Bilirubin, long regarded as a metabolic breakdown product of heme, is now recognized as an endogenous antioxidant with anti-inflammatory and immunomodulatory effects, and thus represents a potential biomarker of disease activity. This master’s thesis aimed to assess serum bilirubin levels in patients with Sjögren’s disease in a Slovenian cohort and evaluate their associations with inflammatory markers, immunological parameters, disease activity, and salivary gland involvement. The study was designed as a non-interventional retrospective analysis of patients treated at the Department of Rheumatology, University Medical Centre Ljubljana, between 2014 and 2024. Baseline total and direct bilirubin values and relevant clinical, laboratory, and imaging disease parameters—including the disease activity index and salivary gland ultrasonography—were retrieved from electronic medical records. The statistical analysis included 130 patients (95% women) with a mean age of 58.2 ± 14.2 years. The median total bilirubin was 8 μmol/L (interquartile range 6–10 μmol/L) and the median direct bilirubin was 2 μmol/L (interquartile range 2–3 μmol/L), corresponding to the lower part of the reference range. Total bilirubin correlated weakly and negatively with erythrocyte sedimentation rate (r = −0.180; p = 0.040) and platelet count (r = −0.215; p = 0.014), whereas no associations were demonstrated with the disease activity index or ultrasonographic salivary gland score. Non-smokers had higher bilirubin levels than smokers or former smokers (p = 0.034). In 99 patients with a follow-up visit after approximately five years, the median bilirubin did not change (p = 0.414), and the change in bilirubin was not associated with a change in the ultrasonographic score. We conclude that serum bilirubin concentrations in this cohort lie in the lower part of the reference range and are associated with weak indicators of systemic inflammation and smoking status, but not with the global assessment of disease activity or ultrasonographic salivary gland involvement. The retrospective design and lack of a control group limit comparisons with healthy populations and support prospective controlled studies with standardized follow-up.
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