Relevance of complement fixing antinuclear antibodies

Area of Science:

• Rheumatology
• Immunology
• Dermatology

Background:

• Connective tissue diseases (CTDs) encompass a diverse range of disorders characterized by varied clinical symptoms, immunological findings, and frequent syndrome overlaps.
• Subacute cutaneous lupus erythematosus (SCLE) and Sjögren's syndrome are CTDs often presenting with complex immunological profiles.

Observation:

• This study investigated eight ANA-negative SCLE cases with anti-Ro (SS-A) antibodies and one complex case exhibiting features of Sjögren's syndrome and systemic lupus erythematosus (SLE).
• The ninth patient, despite negative serum ANA and C-ANA tests, showed positive in vivo complement fixation to bound ANAs in skin biopsy via C + DIF, alongside antimitochondrial antibodies.

Findings:

• Five of eight ANA-negative, Ro(SS-A) positive SCLE patients demonstrated C-ANAs in their serum.
• The complex case revealed both in vivo and in vitro complement-fixing antinuclear antibodies (C-ANAs), indicating their presence despite initial negative serological markers.
• Routine serum tests for the complex case identified antimitochondrial antibodies, while normal skin exhibited weak ANA and strong C-ANA reactivity.

Implications:

• Complement-fixing antinuclear antibody (C-ANA) testing can be a valuable tool for diagnosing CTDs, particularly in ANA-negative presentations of SCLE and Sjögren's syndrome.
• Identifying C-ANAs can improve diagnostic accuracy and potentially guide treatment strategies for patients with overlapping CTD features.
• These findings highlight the importance of considering C-ANA assays in the diagnostic workup of patients with suspected autoimmune connective tissue diseases who present with atypical serological results.