Is it dangerous to have a ANA of 1:640 and elevated sed. Rheumatoligist does not know why.?

Lab values. Individual laboratory results cannot be used out of the context of symptomatic disease and physical findings combined with additional testing. The only way to answer this question would be for an experienced clinician to see you and make a determination of the probable cause for these lab results. ANA elevation may be due to numerous causes, but by itself is not likely dangerous.
+ ANA not= SLE. A + ANA does not equal lupus. It is a blood test that needs to be interpreted in relationship to the clinical setting, other laboratory, imaging, etc. You can see a positive ANA due to connective tissue diseases, allergies, thyroid disease, allergic episodes, etc. We see patients all the time who have a positive ANA every day and no other separate definable medical diagnosis. It is not dangerous.