The extent of the search for prior art to be conducted for the purposes of the European search report is defined in Art. 92 EPC, which states that the EPO shall be drawn "on the basis of the claims, with due regard to the description and any drawings". In principle, and in so far as possible and reasonable, the search should cover the entire subject-matter to which the claims are directed or to which they might reasonably be expected to be directed after they have been amended (Guidelines B‑III, 3.5 – November 2018 version).

In T 708/00 (OJ 2004, 160) the application was refused by the examining division, principally on the grounds that the new claims were not allowable under R. 86(4) EPC 1973. The board stated that the provisions of Art. 92(1) EPC 1973 must not have the effect of limiting the search solely to the subject-matter of the claims, even if this would simplify matters for the search division. Nor does this imply that the sole purpose of the description and drawings is to help interpret the claims. It does, however, mean that the search division has to be discerning in the use of its discretionary power so as to ensure that the search is in complete accordance with the subject-matter of the patent application (see R. 44(1) EPC 1973), i.e. the invention, even if the essential features are not all defined in the claims at the time of search but are disclosed for example in the description or drawings. In the case in hand, the applicants were never informed of exactly what subject-matter is searched, which means that the examining division and board of appeal were also without any information in this regard. It was therefore impossible – since negative proof is out of the question – to establish what subject-matter went "unsearched" under R. 86(4) EPC 1973. This rule therefore had to be interpreted using alternative means such as the preparatory documents or the documents published regarding the adoption of the rule. The board came to the conclusion that amended claims may only be refused on the basis of R. 86(4) EPC 1973 if the subject-matter of the claims filed originally and that of the amended claims is such that, had all claims originally been filed together, a further search fee would have been payable – on top of the search fee payable in respect of the claims actually filed at the outset – in respect of the amended claims, relating to a different invention within the meaning of R. 46(1) EPC 1973. The board noted that R. 86(4) EPC 1973 must be interpreted so as to fairly balance the objective of this rule, namely the EPO's interest in collecting, in return for services rendered, search and examination fees, and the fundamental right conferred by the EPC on the applicant to make amendments at least once to the description, claims and drawings that prove necessary during the grant procedure. The board pointed out that the intention and purpose of R. 86(4) EPC 1973 and the interference with the applicant's fundamental right to amend the application at least once called for a strict interpretation of R. 86(4) EPC 1973 (see also T 141/04). It held that R. 86(4) EPC 1973 did not apply when the applicant had not paid the search fee in respect of a non-unitary invention relating to the originally filed claims in spite of being invited to do so under R. 46(1) EPC 1973. In that case, the application could not be examined further, and a divisional application would have to be filed if protection were sought (see G 2/92, OJ 1993, 591). The boards in T 319/96 and T 631/97 (OJ 2001, 13) made a clear distinction between applying the provisions of R. 86(4) EPC 1973 and a situation where search fees were not paid in spite of an invitation to do so under R. 46(1) EPC 1973.

In T 789/07 the board held that in order to search a claimed feature completely – as required by Art. 92 EPC and in line with the Guidelines – the search division must as a rule ascertain how the feature is to be interpreted in the light of the description and drawings. Only then can it arrive at a reasoned assessment about the amendments likely to be made to the claims during examination proceedings and within the framework of the original disclosure. This interpretation also forms the basis for comparison, with a view to R. 86(4) EPC 1973, with the originally claimed invention or group of inventions. The board also took the view that a claimed feature falling within the scope of the search must be regarded for R. 86(4) EPC 1973 purposes as searched, even if it was not in fact searched in a specific case (see also T 2334/11, T 1679/10, T 345/13, T 1503/13). In T 1679/10 the board added that it would go too far to construe the Guidelines as meaning that it would be sufficient to have one broad independent claim in the application as filed, with no dependent claims, in order to be entitled to a search extending to all the features of a large number of embodiments covered by the wording of that claim.

In T 264/09 the board stated that if a feature which has not been searched and which further defines an element which was already part of a claim which had been searched, is taken from the description and added to this claim in order to overcome an objection under the EPC, e.g. lack of inventive step, this does not result in a claim which has been amended such that it relates to unsearched subject-matter which does not combine with the originally claimed invention to form a single general inventive concept (see also T 2334/11, T 1503/13). The board thereby followed the well-established case law on the interpretation of R. 86(4) EPC 1973 (see e.g. T 708/00, OJ 2004, 160; T 274/03; T 141/04), it being noted that R. 86(4) EPC 1973 has the same wording as R. 137(4) EPC (as in force at the date the examining division took its decision), which, as from 1 April 2010, is included in R. 137(5) EPC. The board noted that the Guidelines were drafted accordingly; these explicitly mention the possibility of carrying out an additional search in connection with the examination of amended claims.