Relevance of complement fixing antinuclear antibodies

Background: Connective tissue diseases (CTDs) are a heterogeneous group of disorders defined by the association of a variety of clinical manifestations with immunologic and other laboratory findings. Overlap of syndromes and aberrant findings appear rather frequently. Methods: Sera of eight antinuclear antibody (ANA) negative, cases of subacute cutaneous lupus erythematosus (SCLE) with antibodies to Ro (SS-A) and a ninth case with clinical and laboratory signs of Sjogren's syndrome and systemic lupus erythematosus (SLE) were tested for complement (C') fixing antinuclear antibodies (C-ANAs). The ninth case was examined in depth by direct immunofluorescence (DIF) and a two-step 'C + DIF' test of biopsies for C' fixation to in vivo bound ANAs, as well as serum tests for C-ANA, ANA, and SCLE markers. Results: Sera of five of the eight ANA negative, Ro(SS-A) positive SCLE cases had C-ANAs. The ninth case, a 50-year-old woman with clinical and laboratory signs of Sjogren's syndrome and SLE, gave a strong positive C + DIF reaction in the skin biopsy for in vivo bound ANAs that fix C', but negative ANAs and C-ANAs in routine serum tests; they revealed antimitochondrial antibodies. Serum tests on normal skin, however, revealed weak ANA and strong C-ANA reactions with in vitro fixed C'. Conclusions: ANA negative cases of SCLE or Sjogren's syndrome may have C-ANAs. A case with Sjogren's syndrome and signs of SLE had both in vivo and in vitro C' fixing ANAs. C-ANA tests can aid in the identification of such cases.