I am not familiar with that test. It looks like it is a type of antibody test (monoclonal antibody). Perhaps he is looking for an autoimmune disease that affects the muscles? You could probably call the lab and ask what it is for.
Is the doctor wanting to confirm that you have myotonia congenita and not another type of myotonic disorder? In MC we generally don't have problems with our heart, but in myotonic dystrophy there are often conduction issues. It's always a good idea to have the echocardiogram even if it's just for comparison later on.