2.5 Closest prior art and its effects on the search 

Sometimes the search division does not find any documents published before the earliest priority date which prejudice the novelty or the inventive step of the claimed invention. Wherever possible, it will then cite in the search report at least that prior art found which discloses a solution to the same subjective problem as the one dealt with by the claimed invention and whose known solution comes technically closest to the claimed solution. This prior art is cited as an "A" document in the search report (see B‑X, 9.2.2).

If it cannot find such a document, the search division will instead cite as the closest prior art a document which discloses a solution to a problem closely related to the one dealt with by the claimed invention and whose solution is technically the most similar to the one in the searched application.

The search division proceeds in the same way where it finds documents which are accidentally prejudicial to the novelty of the claimed invention (cited as "X" documents) but which do not affect inventive step once the application has been amended appropriately, and does not retrieve any other documents prejudicing inventive step.

Where a European application is derived from an international application and undergoes a supplementary European search after entering the European phase (Art. 153(7) – see B‑II, 4.3), the search division sometimes does not uncover any relevant prior-art documents other than the ones already cited by the ISA in its international search report. When this happens, it is permissible for its supplementary European search report not to cite any additional relevant documents (see B‑X, 9.1.4).