GA C4.1.2 4.1.2.

When is it useful to file a demand for international preliminary examination? 
4.1.006
Regardless of whether a demand is filed, applicants will receive a preliminary non-binding written opinion on the patentability of their claimed invention (WO-ISA, IPRP Chapter I) as part of the international search procedure (see points 3.2.005 ff). Therefore, the usefulness of the optional international preliminary examination under Chapter II PCT must be considered carefully for each international application.
PCT Newsletter 4/2010, 8 
5/2010, 8 
4.1.007
International preliminary examination will, in general, have no added value unless amendments and/or arguments under Article 34 PCT are filed. In the absence of such amendments and/or arguments there will be no reason for the EPO as IPEA to come to a different conclusion to that which it reached in its capacity as ISA, as set out in the WO-ISA.
4.1.008
If the ISA has provided the applicant with a declaration under Article 17(2) PCT that no ISR has been established ("declaration of no search"), filing a demand is not recommended, since the EPO as IPEA will not examine claims for which no ISR has been established (see point 4.2.027).
4.1.009
It follows that filing a demand for international preliminary examination is, in general, useful only if the WO-ISA was "negative" and the applicant is of the opinion that his amendments and/or arguments filed for the purpose of the PCT Chapter II procedure may lead to a "positive" IPER. In other words, international preliminary examination gives the applicant the opportunity to respond to any negative finding of the ISA during the international phase, as opposed to drafting and filing multiple responses with the national/regional offices where national phase entry is made. In the Chapter II procedure before the EPO the applicant will have the opportunity to interact with the examiner and file further amendments should objections still persist after the reply to the WO-ISA (see point 4.2.017).
4.1.010
An advantage of using the EPO as IPEA is that a "top-up" search is performed in order to reveal any further relevant prior-art documents, in particular intermediate prior art, which have been published or have become available to the IPEA after the international search report was established (see points 4.2.009 ff).

13 references found.

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EPO Guide for Applicants, part II Int.

EPO Guide for Applicants, part II Int. - C The EPO as ISA and SISA

PCT Articles

PCT Implementing Rules

Offical Journal of the EPO