The thumb rule is to conduct an international search for all international applications. It is standard procedure for you to choose an International Search Authority while filing your international application. Basically, once you submit your application, the ISA will carry out the search in due time. There are presently 15 ISAs. Every Receiving Office allows applicants access to at least one ISA. Some countries allow PCT applicants to choose from more than one ISA.
However, there might be certain conditions when ISA will be unable to conduct a search. In those cases, ISA will issue a declaration stating non-issuance of ISR. ISA can also issue a partial report search if it feels that the application has multiple inventions but the applicant has not paid additional fees to conduct the search for all entailed additional inventions.
Also, make sure that you check out this free e-book: A Quick Guide to Patents.
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