Antinuclear antibody (ANA) tests identify antibodies present in serum that bind to autoantigens present in the nuclei of mammalian cells. Most of these antibodies are IgG, but IgM and IgA have also been detected. The enzyme-linked immunosorbent assay (ELISA) method involves the interaction of these antibodies present in the serum sample with a preprepared antigen, and the addition of an antibody that adheres to this complex and induces a color change; the result is an optical density value (a photometric scale) that is read as positive, negative, or equivocal.
The reference range for antinuclear antibody is negative by ELISA.
If the ELISA method results in an abnormal or equivocal finding, the sample is titered using indirect immunofluorescence (IFA) assays on Hep-2 cells, and any value less than or equal to 1:40 dilution (or < 1.0 IU) is negative.
The frequency of positivity on ANA screening test (on Hep-2 cells) is as follows:
Mixed connective tissue disease: 100%
Drug-induced lupus erythematosus: 100%
Systemic lupus erythematosus: 95%-100%
Sjögren syndrome: 80%
Rheumatoid arthritis: 40%-60%
Normal: Less than 4%