How long after initiation of steroids and immunosuppressive therapy does muscle biopsy remain reliably informative in the setting of suspected inflammatory myopathy?

I don't think there is any definitive, well-validated answer here.

Certainly, obtaining a biopsy within ~2 weeks of initiating therapy, in my experience, has clearly not affected the histopathologic findings that would be characteristic of dermatomyositis, necrotizing myopathy or autoantibody-associ...