None of the autoimmune disease bloodtests are a pass/fail type, they all need to be interpreted in the presence of symptoms and patient history.
Your ANA can change, bloodtest to bloodtest. So your next one may come back low - it sounds like that's what your rheumy's expecting.
And 1:640 is high enough for the docs to consider Lupus but it's not super high. "I dont believe in blood tests... then he proceeded to take 7 vials of blood."
- ROTFL! Yup, that's a rheumatologist for ya. Wait till he says he doesn't believe in x-rays . . . you'll leave the office glowing with radiation.