There are 2 important criterion for the selection of Sterile API’s.
Quality attribute of Sterile API: The selected Sterile API must meet all Quality specifications regarding its Purity, Assay content (Both Chemical and Biological-What ever is necessary),Protein content -In case of Recombinant Biologics, Limits of particulate matter, Solubility charecterestics of a dry powder,Clarity if it is in form of sterile solution, Appearance or colour, Endotoxin limits,Storage conditions, Stability data and Sterility of API along with the container packing integrity. There are no short cuts for this.
Process Consideration: If the manufacturing process is sterile the API taken into the process might be sterile due to the non-avilability of process of rendering the API sterile at the site of Formulation manufacturing. This API might not be able to with stand the sterilization methods as it might lose its labled content or Assay during such process or the API might get destroyed during such process. This is very common in case of Sterile Powder Antibiotics and in case of certain Therapeutic proteins and Biologicals. There are manufacturers who sterilize these API products at the site of Forumaltion processing by special means with special provisions, but they are limited to certain specific products.