Background: Anti-C1q autoantibodies can disrupt normal complement function, contributing to the formation of pathogenic immune complexes and end-organ damage. Although their role in systemic lupus erythematosus (SLE) is well-established, their prevalence and clinical relevance across a broader spectrum of diseases remain insufficiently characterized.

Objectives: This study aimed to investigate the distribution of abnormal anti-C1q antibody levels in the population and examine their associations with age, sex, and specific clinical subtypes.

Methods: This retrospective study included patients who underwent anti-C1q antibody testing at our hospital between September 2020 and September 2023. The primary outcome was the prevalence of abnormal anti-C1q antibody levels (>10U/mL) categorized by patient sex, age, and disease diagnosis. One-way and two-way fixed-effect models were used to assess associations between odds of abnormal antibody levels and demographic factors. Multivariate logistic regression was to performed to identify disease-specific correlates.

Results: Among 15,363 patients (median [IQR] age, 38 [28–53] years; 79.0% female; >50% aged <40 years) representing 67 distinct diagnoses, 7.9% showed abnormal anti-C1q antibody levels. Female sex and younger age were associated with higher median anti-C1q antibody levels and a greater proportion of abnormal results. SLE subtypes exhibited the highest prevalence of abnormal anti-C1q levels, with SLE without severe complications (22.7%) and lupus nephritis (29.9%)being most obvious. Additionally, lupus hematologic and encephalopathic manifestations were associated with elevated antibody levels. Both univariate and multivariate analyses indicated that male sex and younger age were significantly associated with increased odds of abnormal anti-C1q antibody levels.

Conclusion: Elevations in anti-C1q antibody levels extend beyond SLE and are influenced by both demographic factors and specific disease phenotypes. Male sex and younger age emerged as significant predictors of abnormal anti-C1q antibody status. Our findings underscore the potential utility of anti-C1q antibody testing for improving diagnostic precision and risk stratification across a wide range of clinical conditions.

REFERENCES: NIL.

Acknowledgements: NIL.

Disclosure of Interests: None declared.

© The Authors 2025. This abstract is an open access article published in Annals of Rheumatic Diseases under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). Neither EULAR nor the publisher make any representation as to the accuracy of the content. The authors are solely responsible for the content in their abstract including accuracy of the facts, statements, results, conclusion, citing resources etc.