Navigating the labyrinth of patent citations

Key concepts at a glance

Patent citations are a powerful source of information not only for applicants and IP offices, but also for patent searchers. Hidden away on the last pages of search reports, these rather unassuming references are occasionally overlooked. However, they are far more interesting than their dry descriptions suggest. In this article, we will delve a little deeper into the labyrinth of patent citations and provide an overview of some of the most important key concepts.

Just as sources must be cited in research papers, patent citations are used to list patent documents and other sources that are relevant to the content of a patent application. They can be submitted by the applicant, the patent examiner or even a third party, for example during an opposition or invalidation procedure.

Huge amounts of data

The huge volume of patent applications at the EPO results in an even larger volume of citations. In 2023 alone, a whopping two million documents were cited in EP publications: around half of them by applicants themselves, and half by EPO examiners during search and examination procedures.  Additionally, some 36 000 citations were made during opposition proceedings.

Looking beyond EP publications, the EPO also maintains its REFI database (see link below under “Further information”) for worldwide citation data. REFI covers around 50 million citations from more than 40 patent offices worldwide (see link to coverage table below). You can access this data via the EPO’s patent information services such as Espacenet, PATSTAT or the new Technology Intelligence Platform.

For a detailed overview of citation data provided by IP5 offices (China, Japan, South Korea, US and EPO), see the EPO's 'Coverage, codes and statistics' webpage (link listed below under “Further information”). This table shows not only the total number of citations provided by each office, but also the publication levels at which the different types of citations are available in the bibliographic records:

Citation categories

As highlighted above, a large proportion of citations are submitted during the search and examination procedure. The primary function of citations made by EPO examiners is to make the applicant aware of existing technology relevant to their invention, which the examiner takes into account when deciding whether to grant a patent. Think of it as helping to document the boundaries for deciding on novelty and inventive step.

In the search report, all cited documents are assigned to a category indicating the relevance of the document cited by the examiner. Each category is identified by a letter, which is usually shown in the first column of the citation sheet. Some of the most important categories are listed below:

X          Particularly relevant document. The cited document alone shows that the claimed invention cannot be considered new or to involve an inventive step.

Y          Particularly relevant document, but only if combined with another cited document of category Y.

A          Technological background: The document represents the state of the art but does not prejudice the novelty or the inventive step of the claimed invention. 

Category X citations will have the greatest impact on the outcome of the examination procedure, particularly if the X document relates to an independent claim of the invention. For more information, please refer to the EPO’s Examination Guidelines, Section 9.2 Categories of documents (see link below).

Forward and backward citations

Several databases show a patent’s cited documents mentioned above as well as so-called citing documents. While cited documents are referred to as “backward citations”, citing documents are also called “forward citations”:

• Backward citations or cited documents are documents that are cited by a specific patent document during any of the procedures before a patent authority or by the applicant.
• Forward citations or citing documents are patent documents that cite a specific patent.

In other words: A 'backward citation' is the term used for a traditional citation: it is the document that was published earlier, and which appears on the newer document’s front page. In turn, the newer document is called the 'forward citation' or 'citing document'.

If Patent A (1993) is cited by Patent B (2003), then:

• Patent A is a backward citation of Patent B
• Patent B is a forward citation of Patent A

Obviously, forward citations cannot appear on a document’s front page, since no one can see the future, and patents are published at time of issue. However, forward citations can easily appear on the patent record in an electronic database, and most search databases allow both backward and forward citation searching.

We hope that this article provided an orientation for navigating the labyrinth of patent citations, and that it will be of help for you when accessing this data in the EPO’s search tools. 

Keywords: Patent citation, REFI, search report, reference, forward citation, backward citation