While the antinuclear antibody is not a specific test for lupus, it is sensitive and does detect the antibodies that are present in 97 percent of people with the disease.

Pattern on the antinuclear antibody-HEp-2 test is a critical parameter for discriminating antinuclear antibody-positive healthy individuals and patients with autoimmune rheumatic diseases.

January 12, 2011 — Up to 13% of healthy people test positive for antinuclear antibodies (ANA), which are associated with autoimmune disorders such as rheumatoid arthritis, Sjögren's syndrome ...

While most people with systemic lupus will have a positive ANA, you are correct that some people will have a positive ANA test without any indication of illness.