An Applicant of an International Patent Application is sometimes given the choice of having a search done on the invention at a patent office other than at the Receiving Office. The patent office which performs the search is called the "International Searching Authority." Indeed, not all Receiving Offices are qualified to act as International Searching Authorities, and therefore International Applications filed at such Receiving Offices are regularly referred to a different patent office than where the International Application was initially filed for purposes of having a search of the prior art made.

Even when the patent office functioning as the Receiving Office also has the status of an International Searching Authority, agreements may have been entered into which allow another patent office to serve as the International Searching Authority at the election of the Applicant.

For example, nationals or residents of the United States may opt to have the International Search carried out at the European Patent Office (EPO) or the Korean Intellectual Property office (KIPO)*. If the Applicant originally filed a national patent application in the US and thereafter filed the International Patent Application, they will most likely receive the results of the search done on the national application before a decision to enter the National or Regional Phase must be made at thirty months from the priority date. Thus at the time the applicant makes a decision to enter the National or Regional Phase of a International Application and to thus incur the expense associated with that decision, they can have the benefit of two prior art searches, one done at the US patent office (based on their national patent application) and the other done at either the EPO or the KIPO (based on their PCT application), to aid them making that decision.

Thus, applicants in International Applications can often have the benefit being able to review the results of two prior art searches done by major patent offices before plunging ahead with filing foreign patent applications.

* Effective January 1, 2006