I got a shot of cortisone 5 or 6 days before labs for polymyositis, will it effect the results?

Yes: This is a loaded question--if you are flaring with your disease, these shots may reduce your flare and the labs will reflect this. If there is no disease activity, then there may not be a difference. The type of injection also matters. Higher doses will have a more significant influence on your body than lower doses. Also the type of cortisone used will affect how long it lasts in your system.