A 38-year-old man complains about mild pain and muscle weakness of shoulder and pelvic girdles and back that has been progressing for the last 3 weeks. He has also significant problems with getting up, going up and down the stairs and shaving. It is suspected that the patient is suffering from dermatomyositis. Blood count: Hb - 114 g/l, leukocytes - 10,8*109/l, eosin - 9%, ESR - 22 mm/h, C-reactive protein (++). Change of the following laboratory factor will be of the greatest diagnostic importance: