Anti-Nuclear Antibodies are a part of the immune system that specifically attacks the nucleus of cells.  In a healthy body, they are a normal part of the body's process to remove dead and dying cells from the bloodstream.  However, an excess of anti-nuclear antibodies in a patient is usually an indication of an autoimmune disease.  Such high concentrations are almost always found in patients with lupus, but can be the result of other autoimmune conditions such as Sjogren's syndrome, rheumatoid arthritis, and scleroderma.  The concentration can also be found in patients suffering from Addison's disease or diabetes mellitus.  However, about 2% of the population has high ANA concentrations without suffering from any autoimmune disesae at all.

Testing ANA concentration is done by titration, a process of diluting blood until ANAs cannot be detected by the test on the resulting solution.  In a normal patient, ANAs cannot be detected when the solution is less than about 2.5% blood.  However, in many patients with lupus, ANAs can still be detected at concentrations of less than 2% and even in solutions which are only about 1% blood. 

Anti-nuclear antibody at Wikipedia

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