I personally have never had the test done because I have never been diagnosed with Lupus, but please visit this website to update yourself on all tests concerning Lupus. Your doctor is right, it is very hard to diagnose as it looks like (from blood work) many other auto immune diseases.
You mean ENA test by the way, and all Lupus blood tests are explained on this site:
Lupus site(SLE)- information on lupus