A:

The anti-nuclear antibody (ANA) is the most common blood test a doctor orders when he or she suspects lupus (systemic lupus erythematosus or SLE). Almost everyone with lupus has a positive ANA. But many, especially women, have a positive ANA and do not have lupus.

If there are symptoms and signs of lupus, the ANA is a good first screening. If the ANA is not found, it’s unlikely that it’s SLE since 99% of people with it have a positive test.

On the other hand, a positive ANA test does not mean a person has lupus. In fact, lupus cannot be diagnosed just on the basis of positive ANA alone.

The amount of ANA also matters. For instance, a low level (a low titer) of less than 1 to 160, means lupus is unlikely. If the ANA titer is 1 to 160 or higher, there can be other reasons for a positive ANA test other than lupus. Your doctor can order specific ANA tests that better predict the presence of lupus. A positive test for anti-Smith or anti-double-stranded-DNA antibodies strongly suggests SLE.