What could a negative ana and positive rnp antibodies mean? i have a history of chronic rheumatological symptoms, but my dr is not convinced its mctd.

Lab problem: The ANA screening test is fraught with pitfalls, and if you have a way-high titer of U1-RNP on the tube test and symptoms / signs of mixed connective tissue disease (which is a rough thing) but ANA is negative, perhaps there's a problem with the substrate. Your physician will want to call the lab's pathologist -- he / she would be interested in knowing. Best wishes.