what about Privigen? Gammunex?

I was receiving Octogam 5% from November-April 2009 for CIDP. The infusion center changed to Privigen 10% & I have getting this up until now. My doctor said Privigen is less expensive compared to other IVIG’s. I am getting weaker (I am getting 25 gms IVIG every 2 weeks) and my doctor says it may be the brand of IVIG that I am getting. Is there a difference between Privigen, Gammunex, Gammaguard? He said Gammunex is targeted for CIDP. But the infusion center I go to only offers Privigen. I don’t think the infusion center will change to Gammunex even though the doctor orders it for me. I am confused as to how infusion centers pick the brand of IVIG to use for patients.