From the Guidelines

Antinuclear Antibody (ANA) is a type of autoantibody that is fundamental for the diagnosis of autoimmune diseases.

ANA are detected by indirect immunofluorescence assay (IIFA) and are considered positive when they are detected at a titer ≥1:80 by the indirect immunofluorescence test on HEp-2 cells (IFI) 1.
The detection of ANA is the first level test for laboratory diagnosis of systemic autoimmune rheumatic diseases (SARD) and is primarily intended for diagnostic purposes, not for monitoring disease progression 1.

The IIFA is the reference method for ANA screening, and alternative assays can be used while keeping in mind that false negative and false positive ratios of these methods may be different 1.
Diagnostic laboratories should specify the methods used for detecting ANA when reporting their results, and tests based on a restricted mixture of defined nuclear antigens should not be referred to as ANA test or ANA screen 1.

A proper ANA–IIFA is dependent on reagents, equipment, and other local factors, and the screening dilution should be defined locally, with an abnormal ANA being the titre above the 95th percentile of a healthy control population 1.
In the case of a positive ANA test, it is recommended that the pattern and the highest dilution to demonstrate reactivity be reported, and ANA–IIFA patterns should be reported according to standardized terminology 1.

ANA are associated with a variety of systemic rheumatologic disorders and are relatively frequent in healthy individuals, with a specificity of 74.7% for systemic lupus erythematosus (SLE) when detected at a titer ≥1:80 1.
The use of ANA detection as an entry criterion in the SLE classification can be considered a strategic move, but it is necessary to insist that this is a classification criterion of limited use in diagnosing or screening 1.

From the Research

Antinuclear antibodies (ANAs) are a diverse group of autoantibodies that recognize nuclear macromolecules and their complexes 2.
ANAs are immunoglobulins or antibodies that bind to one or more antigens expressed within the nucleus of human cells 3.
They are directed against autologous cell nuclear and cytoplasmic components, and can be detected in circulation 4.

ANAs represent key biomarkers in the evaluation of rheumatic diseases, most prominently systemic lupus erythematosus (SLE) 2, 5.
A positive ANA test may also be seen with non-autoimmune inflammatory diseases, including both acute and chronic infections 6.
ANA testing is commonly performed in the clinical setting to help diagnose patients with clinical symptoms suggestive of possible autoimmune diseases 6, 3.

ANAs can be assayed by various techniques, with the fluorescent ANA assay often viewed as the gold standard 2.
Indirect immunofluorescence (IIF) is recommended as a screening test, and positive samples should be confirmed by ELISA, western blot, or other techniques 4.
The pattern and titer of ANA detected by IIF must be evaluated, and specificity characterization (reactivity against extractable nuclear antigens [ENA], dsDNA, etc.) is useful for diagnosis and follow-up of patients with autoimmune diseases 4.