7yrs on prednisone & methotrexate + for "resistant ra" recent elevated jo1, histone, muscle pain. How to confirm if polymyositis w/o stopping meds?

Mucle testing. Inflammatory myopathy, including polymyositis, is best diagnosed and treated by a trained rheumatologist. Symptoms are proximal muscle weakness. Abnormal lab tests include elevated muscle enzymes (CPK, AST/ALT); positive antibody tests, including Jo-1; abnormal EMG/nerve conduction test; edema seen on an MRI of proximal muscles; muscle biopsy showing destruction and regeneration of muscle cells.
See below. The diagnosis of polymyositis would be based iniatilly on the presence of weakness paricularly proximal muscles. The next step would be to check blood tests for muscle inflammation including a CPK and aldolase. If thoes are abnormal the next step is a nerve and muscle test called an emg/ncv. If that is abnormal the most definitive test is a muscle biopsy so a jo-1 would not make the diagnosis.
Both drugs work both. Both drugs are effective in polymyositis. Your jo-1 could be specious so you need evidence for muscle inflammation.A serum CPK and myoglobin would best reflect the activity of a "myositis, " should one exist. Also, if you have ra, your ccp antibody is positive 98% of the trime. Finally, some patients have two diseases, always a possibility.