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 Sjögren’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 Sjögren’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 Sjögren’s syndrome may have C‐ANAs. A case with Sjögren’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.