Assume you are referring to your cpk or ck number. 460 is elevated but not terribly high. I have pm and mine was 13000 and I have talked to those who had 100-200,000. Cpk is not a direct test for pm but it is an indication of inflammation in your muscles...spillage of creatine phosphokinase out of the cells. This is a complex subject and diagnosis of myositis is made by clinical symptoms, labs, emg, mri, and often it takes a muscle biopsy to confirm the diagnosis.
If you have pm there is a distinct pattern of weakness..symmetrical, weakness around the hip and shoulder girdles. The first signs are difficulty getting out of a chair, off the toilet, going up stairs, raising your arms above your head, and sometimes swallowing.
Like all AI diseases myositis can be mild to life threatening and every case is unique. Prednisone is the 1st line of treatment but does not always work. It is important to have a doc who is experienced treating AI diseases like lupus and myositis.
Let me know if you have more questions. I have pm and lupus and had an extreme case but survived and have made a great recovery.