An International Search Authority (ISA) is responsible for carrying out the international search of a PCT application. Also, the applicant has the liberty to choose the ISA at the time of international filing. Sometimes, you can select more than one ISA as well. Ultimately, the ISA needs to carry out a thorough and deep patent search internationally. Then, it needs to provide the applicant with the results in an International Search Report (ISR). Along with the ISR, the ISA also needs to hand out a Written Opinion (WOSA) which determines the patentability of the application.
Hence, the ISA indeed has a very important responsibility of conducting an elaborate and deep international search of a PCT application.
Also, do make sure that you check out this free e-book: A Quick Guide to Patents. It deals with basics of patent procedures.
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