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Evaluating patients suspected of having a systemic rheumatic disease
Measurement of antinuclear antibodies (ANAs) in serum is the most commonly performed screening test for patients suspected of having a systemic rheumatic disease, also referred to as connective tissue disease.(1) ANAs occur in patients with a variety of autoimmune diseases, both systemic and organ-specific. They are particularly common in the systemic rheumatic diseases, which include lupus erythematosus (LE), discoid LE, drug-induced LE, mixed connective tissue disease, Sjogren syndrome, scleroderma (systemic sclerosis), CREST (calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) syndrome, polymyositis/dermatomyositis, and rheumatoid arthritis.(1)
The diagnosis of a systemic rheumatic disease is based primarily on the presence of compatible clinical signs and symptoms. The results of tests for autoantibodies including ANA and specific autoantibodies are ancillary. Additional diagnostic criteria include consistent histopathology or specific radiographic findings. Although individual systemic rheumatic diseases are relatively uncommon, a great many patients present with clinical findings that are compatible with a systemic rheumatic disease and large numbers of tests for ANA are ordered to eliminate the possibility of a systemic rheumatic disease.
See Connective Tissue Diseases Cascade Test-Ordering Algorithm in Special Instructions, also see Optimized Laboratory Testing for Connective Tissue Diseases in Primary Care: The Mayo Connective Tissue Diseases Cascade in Publications.
< or =1.0 U (negative)
1.1-2.9 U (weakly positive)
3.0-5.9 U (positive)
> or =6.0 U (strongly positive)
Reference values apply to all ages.
A large number of healthy individuals have weakly-positive antinuclear antibody (ANA) results, many of which are likely to be clinical false-positives; therefore, second-order testing of all positive ANAs yields a very low percentage of positive results to the specific nuclear antigens.
A positive ANA result at any level is consistent with the diagnosis of systemic rheumatic disease, but a result > or =3.0 U is more strongly associated with systemic rheumatic disease than a weakly-positive result.
Positive ANA results >3.0 U are associated with the presence of detectable autoantibodies to specific nuclear antigens. The nuclear antigens are associated with specific diseases (eg, anti-Scl 70 is associated with scleroderma) and can be detected with second-order testing.
Some patients without clinical evidence of an autoimmune disease or a systemic rheumatic disease may have a detectable level of antinuclear antibody (ANA). This finding is more common in women than men, and the frequency of a detectable ANA in healthy women >40 years old may approach 15% to 20%. ANA may also be detectable following viral illnesses, in chronic infections, or in patients treated with many different medications.
In a study performed in the Mayo Clinic immunology antibody laboratory, >75% of patients with a systemic rheumatic disease had antinuclear antibody (ANA) results >3.0 U and the positive predictive value of these results for a systemic rheumatic disease was >85%. Weakly positive ANA results were not a strong indicator of systemic rheumatic disease. The likelihood of finding an autoantibody to a specific nuclear antigen on a second-order testing increased directly with the level of ANA: 92% of sera that had detectable autoantibodies on second-order testing had an ANA level >3.0 U.(2)
An ANA result of > or =3.0 U is the cut-off for Mayo Medical Laboratories CTDC / Connective Tissue Disease Cascade, Serum, a test algorithm designed to evaluate patients with signs and symptoms consistent with connective tissue diseases and the preferred initial test for these patients.
Results of tests for ANA performed by ELISA in the immunology antibody laboratory show that ELISA and traditional indirect immunofluorescence methods for ANA are substantially equivalent.
1. Kavanaugh A, Tomar R, Reveille J, et al: Guidelines for use of the antinuclear antibody test and tests for specific autoantibodies to nuclear antigens. Pathol Lab Med 2000;124:71-81
2. Homburger HA, Cahen YD, Griffiths J, Jacob GL: Detection of antinuclear antibodies: comparative evaluation of enzyme immunoassay and indirect immunofluorescence methods. Arch Pathol Lab Med 1998;122:993-999