Clinical and Laboratory Parameters in Primary and Secondary Sjögren’s Disease and Sicca Patients: A Croatian Cross-Sectional Comparative Study

Background/Objectives: Sjögren’s disease (SjD) is a chronic autoimmune disorder characterized by a wide range of clinical manifestations, often leading to delayed diagnosis. This study aimed to evaluate and compare the clinical and laboratory characteristics of patients with primary and secondary SjD and those with sicca symptoms without SjD. Methods: This comparative cross-sectional study included 84 participants enrolled between 2019 and 2024: 27 patients with primary Sjögren’s disease (pSjD), 4 with secondary Sjögren’s disease (sSjD), and 53 with sicca symptoms without SjD (non-SjD/NSjD). Primary SjD was diagnosed according to the 2016 American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) classification criteria. Results: Compared with NSjD patients, those with pSjD and sSjD had significantly lower unstimulated whole saliva (UWS) and stimulated whole saliva (SWS) flow rates (p < 0.001), a higher prevalence of antinuclear antibodies (ANA) (twice as frequent) (p = 0.005), and higher frequencies of anti-SSA/Ro60, anti-SSA/Ro52, anti-SSB/La, and rheumatoid factor (RF) positivity (p = 0.002, p = 0.003, p = 0.005, and p = 0.019, respectively). In addition, SjD patients had higher EULAR Sjögren’s Syndrome Disease Activity Index (ESSDAI) scores (p = 0.026) and more frequently demonstrated a focus score (FS) ≥ 1 on minor labial salivary gland (MLSG) biopsy (p < 0.001). Serum vitamin D levels were lower in pSjD patients than in NSjD patients; however, the difference was not statistically significant (57.8 ± 20.4 vs. 70.6 ± 18.3; p = 0.259). Conclusions: Patients with pSjD exhibited distinct clinical and laboratory characteristics compared with those with sicca symptoms without SjD. Sialometry, anti-SSA antibodies, and MLSG biopsy were identified as the most important diagnostic tools for differentiating SjD from NSjD.