A positive ANA blood test indicates that a person's immune system has launched a direct attack on its own tissue, according to the Mayo Clinic. This is also known as an autoimmune reaction. A person does not have to be acutely ill to have a positive ANA blood test.
Antinuclear Antibodies, known as ANA, are different from the normal antibodies the body produces to attack infections, notes the Mayo Clinic. Antinuclear antibodies attack the body's own cells, specifically going after each cell's nucleus. A positive ANA test cannot give a particular diagnosis, but it can be used to rule out certain diseases or conditions.