Sophie, there are two parts to an ANA test.

The first is the titre (the level of anti-nuclear antibodies in the blood). This is measured using an ELISA (an immunoassay) - the titre is expressed as a ratio (1:160, 1:320, etc). The higher the titre the more anti-nuclear antibodies are present in the blood.

The second is the staining pattern - which parts of a nucleus the anti-nuclear antibodies react with.

Many people with no other signs of an auto-immune disease have an unusually high ANA titre. For that reason an ANA test is regarded as confirmatory rather than diagnostic.

This is discussed from a clinical perspective here http://www.racgp.org.au/afp/2013/oct...antibody-test/.