It is a test that is performed by applying the patient's serum to a slide which has been layered with tissue culture cells- usually a cell line called Hep2 (originally derived from a patient with an epithelial cell tumor). If the serum has antibodies that react with cell nuclei they will "stick" to the cells and can be detected with a second antibody - labeled with a fluorescent marker.  When viewed under a microscope, illuminated with an ultra-violet light, the

Antinuclear Antibody Antibodies are seen as blobs of green fluorescence - see picturesbelow.
 

This is what is meant by a positive ANA.  Each laboratory has to determine how much the patient's serum must be diluted before it is reacted with the cells - as "neat" serum often gives a positive ANA test. Thus a positive test is defined as the lowest dilution that would be expected to produce green fluorescence in a patient with disease. This is usually referred to as a titer. For instance, a titer greater than a dilution of 1 in 40 might be the dilution separating a normal serum from a diseased serum. This is usually seen on the lab report as Positive > 1:40. The higher the titer, the more likely the patient has an immune disease such as lupus (SLE).

 Do you have lupus?

The most important thing you need to know about a positive ANA is: it doesn't mean you have lupus.

This is because there are many other causes of a positive ANA.  However a negative ANA
virtually rules out the possibility of having lupus.  The possible causes of a positive ANA are given in the table to the right. Importantly some normal individuals have a positive ANA - this is more likely as you age and if you are female.

The diagnosis of lupus is based on the finding of 4 or more findings out of 11 features. A positive ANA is just one of these findings. Unfortunately some fibromyalgia patients are incorrectly diagnosed as having SLE.