Validity Searching Best Practices

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A validity search is a search that is determined by the selected claims of an issued patent. Validity searches might be required for a number of reasons. Sometimes, the validity of a patent is brought up during litigation. Oftentimes, a validity search is requested even when no lawsuit has been filed, to identify potential attackers and/or references that challenge the presumption of validity of a subject patent.

A validity search also helps with the valuation of a patent. If the searcher discovers closely related prior art that may cast doubt on the validity of the subject patent, the patent may be considered "weak." On the other hand, if the search does not discover these other documents, the subject patent may be considered "strong." This kind of investigation plays in important role when licensing agreements or other royalties are being negotiated between the subject patent holder and a 3rd party interested in practicing its claimed subject matter.

Obstacles Facing the Searcher

One important consideration during a validity search is claim interpretation. Because validity searches are performed on patents that have already been examined and allowed, a broad interpretation of the allowed claims is necessary to find further relevant art. It is absolutely essential for the searcher to give the selected claims the broadest reasonable interpretation. Furthermore, this interpretation must be discussed and clarified with the search requester. Even if such art does not seem to constitute a direct challenge to the claims, it may still form the basis for a legal argument against validity. Successfully defining the scope of a validity search usually requires a strong understanding of the current state of the technology field, as well as some creativity when identify analogous technologies that may also fit into the claim limitations. A step in the specific shows an example of dividing a claim into its particular limitations; this activity can help the user in his or her quest to achieve the broadest possible interpretation. It must be stressed, however, that the interpretation of the claims should also be discussed with the search requester (a patent attorney), and agreed-upon prior to the start of the search. As in all patent searching, the searcher should get as much direction as possible from an attorney, and the task of interpreting any claims should fall directly to an attorney.

Another consideration in validity searching is determining the search cut-off date; ideally, this very important date should be agreed upon by the searcher and search recipient. Put simply, the search cut-off date should be determined to encompass any prior art that might defeat the subject patent's validity. This date is dependent on the national laws in the issuing country from which the subject patent originates. There are a number of legal concerns that dictate what cut-off date should be used for a validity search; however, in all cases, this date must be determined by a qualified attorney.

Sometimes the term "critical date" is used to refer to this search cut-off date; however, in patent law, the phrase critical date has a distinct meaning, and it does not always correlate to the search-cut off date that a patent attorney may request.

Oftentimes, the search requester will designate a search cut-off date of 3 to 5 years after the filing date of the patent to be searched. This is especially useful in certain emerging technologies, where searchers may find highly relevant references published after the filing date of an early seminal patent under validity investigation. Tracing their origins back to an early obscure conference proceeding or other hidden source can form the basis for a legal argument against validity. However, searchers should also bear in mind that the number of publications for a quickly moving technology might skyrocket after a certain seminal publication in the technology’s history, meaning that a post-filing-date search could swamp the searcher with too many useless references. When this happens, searchers should consult with the search requester to determine whether it is appropriate to move the search cut-off date back to an earlier date.

A special obstacle can arise when internet publications are found containing information that appears relevant to a validity investigation. Any publicly available information can be used to make a case against validity, but there is a need to prove that the information was in fact available before the effective filing date of the patent document. One way to do this is to use internet archiving services, which have been crawling the web and making date-stamped copies of web pages. The most well known of these is the Internet Archive (also called the Wayback Machine), available here. This service will not index some pages, for example, those pages that are marked with a robots.txt file to discourage web crawlers, or “orphan” pages that are not linked by any other web pages on the net. Still, there is a chance that technical information publicly available on the web can be date-stamped using this resource.

Another useful tool in a validity search is the patent's prosecution history (also sometimes called a "file history"). A prosecution history is a record of all correspondence between a patent applicant and the patent office that examined the application. It may contain a search report filed by the examiner, which can be a helpful starting-off point for the searcher. It will also often contain a reasoned statement written by the Examiner considering the prior art found during the search, and explicitly describing the novel claim limitations that allowed the patent to issue in consideration of past inventions (in the US, this document is called the Reason for Allowance). Because it states exactly which claim limitations were not found in the prior art, this document is sometimes helpful to determine the focus of validity investigations, although the exact search strategies and claim interpretations should always be controlled by a qualified patent attorney.

Locating the prosecution history can be a tricky business. In the US, some newer patent prosecution histories may be accessed online via the USPTO Public PAIR service. A prosecution history for a newer document may be available as an Image File Wrapper (or IFW), meaning it is fully available online in PDF format; older documents may need to be ordered from a prosecution history service, where employees physically retrieve the records and make copies. Similarly, the EP maintains a system for accessing prosecution histories online, at the EPO's Register Plus service, although not all EP prosecution histories are available this way. The situation gets even trickier when a prosecution history is needed from other patenting authorities. Some patent offices allow physical inspection of their prosecution histories, but do not allow them to be copied. In addition, some patent offices may destroy their records due to lack of storage space. The age and issuing authority associated with a patent document may determine whether or not this valuable resource can be found.

Searching Patent Documents

A validity search should encompass the entire body of potential prior art that could have been used to reject the original patent application. (However, due to the legal complexities involved in what material can be used to reject patent claims, the "search cut-off date" should always be determined by a patent attorney.) To meet these requirements, search tools selected for a validity search should have extended, reliable coverage in US and major non-US full text collections, as well as a complete worldwide bibliographic and family collection from at least one of the two major sources, the EPO’s INPADOC/DOCDB file and the Derwent World Patents Index. Most commercial patent search tools, along with the free USPTO EAST system in Alexandria, VA, will fit these criteria, although users should bear in mind that the more comprehensive the coverage is, the better the search will be. Free tools such as the EPO’s esp@cenet or Google Patents should probably not be used as primary sources, but can serve as useful supplementary sources of information, such as for free patent PDF downloading.

For validity searching in older technologies, specifically the mechanical arts, it is very advisable for searchers to select a data source with a complete collection of US full text patent data. In the mechanical arts, it is possible for a current idea to actually appear in the patent literature far earlier than 1976, the date at which many US full text collections begin in electronic sources. MicroPatent PatentWeb, Thomson Innovation, LexisNexis TotalPatent, and Google Patent Search are examples of sources which provide complete US full text backfile data.

Citation searching is a valuable tool during any search effort, but because a validity search always starts with an issued patent, there should always be an initial investigation into the patent art cited by the examiner. The searcher should attempt to get an idea of the closest art found during the initial search, to understand which claim limitations were discovered by the examiner, and which were not found, allowing the patent to issue. (The patent file history should also be consulted, whenever possible, to gain an understanding of the reasons for allowance. For US patents, this can be done using Public PAIR, and for EP patents, using the EPO's Register Plus service.)

The essential features in any full text patent search tool to be used for validity searching should include:

  • Highest quality data possible
  • As much backfile data as possible
  • Efficient citation search features
  • The ability to limit the search by date using publication, application or priority date

Searching Non-Patent Literature

A validity search must also include a non-patent literature search, encompassing any document published before the search cut-off date. (Due to the legal complexities involved in what material can be used to reject patent claims, this date should always be determined by a patent attorney.) Recommended non-patent literature sources for various technical disciplines can be found in their respective best practices articles. Both large compendiums of information and smaller resources such as individual journals, books, and even web pages can be sources of relevant prior art during this type of in-depth investigation.

Validity searches by nature are intended to be more extensive and in-depth than the search which was performed by the examiner who issued the patent. For that reason, validity investigations often require searchers to consult obscure, unusual, and remote sources of potential prior art. Failing any positive hits from major online sources, it is not unusual for searchers in the most pressing legal cases to utilize major libraries, such as the Library of Congress (US) or the National Library of Medicine (US), to conduct investigations manually. Failing the ability to go to a nationally recognized source, searchers might consider gaining access to university libraries in their area.

Specific Search Strategies

These search strategies are examples of specific best practices that can be applied during the course of an infringement search. These are steps to be taken in addition to accepted general search practices that apply to all searches. For a more general progression of search steps, please see the General Searching best practices article.

  • Develop the search features by making each limitation of the claims its own feature. Through discussion with the search recipient, identify the various limitations which are likely to be the most difficult to find in the prior art. (Usually, the searcher must ensure that every limitation of the selected claims is included in the search features. However, the purpose of breaking the claim into its component features is that it will be easier to find pieces of the claim, rather than the entire claim in entirety.) Because examiners can combine multiple patent references to prove non-obviousness, a validity searcher must look for individual “pieces” of a claimed invention rather than focusing on the whole.

To illustrate the process, if a validity search were to be conducted on claim 1 of US 6,603,868 B1, shown below, the search would begin by identifying these five subject features.

A claim is broken down into five discrete features.

  • Agree on and clarify the broadest reasonable interpretation of claim limitations with the search recipient. Claims in a validity search should always be given the broadest reasonable interpretation; a patent attorney should always be consulted during this process. This step is crucial for searchers to fully understand what to include and exclude in the search results.
  • Identify keywords from the claims of the patent. A patent drafter acts as a lexicographer for the patents she drafts. She can pick words she wants to use and lay stress on specific words. The reverse is also true; she may avoid words she considers less important. A neat way to start an invalidity search is to pick keywords based on the patent drafter’s focus. Generally, a patent drafter would use important keywords in the claims and use them repeatedly with due antecedent basis applied. A searcher can tap this resource and pick out words that have antecedent basis applied to them. This will ensure that the searcher begins the search on the right track, making the initial searches highly focused.
  • Establish a search cut-off date with the person requesting the search. Due to a number of legal complexities involved in determining what constitutes prior art and can be used to challenge validity, this date should always be determined by a patent attorney. A common range will be 3-5 years after the filing date of the subject patent.
  • If possible, review the file history of the subject patent. The patent prosecution is a tool that can provide some extra help and useful clues to the validity searcher. Firstly, the examiner's original search report, including the field of search and relevant results found by the search, is often contained in the prosecution history. Secondly, the prosecution history can provide some answers these two questions: why did the examiner allow the patent application? What material was the examiner unable to find in the prior art? Ideally, in the US, the patent prosecution history will contain a copy of a special document known as the examiner’s Reason for Allowance, outlining why the application was allowed to issue as a patent. However, it is common for this information to be missing from the prosecution history. When this occurs, searchers should review the other documents within the prosecution history (documents with names such as Applicant’s Remarks, Claim Objections, and/or Amendments to the Claims) to find additional clues.
Searchers should note that any conclusions drawn from examining the file history should be discussed with a patent attorney before they are used to direct the search.
Scanned US prosecution histories can be found via the USPTO Public PAIR website (, while EP file histories are available from the EPO's Register Plus service ( For non-US or EP patent documents, the file history may need to be ordered via proxy from the issuing patent office, if the search time budget allows. Certain offices will not even allow photocopies to be made of their file histories, and in extreme cases this may require the searcher (or a proxy) to visit the original copy and make notes by hand to summarize the examiner's decision.
  • When reporting the results of a validity search, consider using a feature matrix to note key subject areas addressed by each reference. Always relate the results to the claimed subject matter. (See the General Searching best practices article for an example of a feature matrix).
  • When reporting your search results, do not offer any opinion about the validity of the patent in question. Searchers should remember that their job is to find references that may be relevant to a particular validity case. However, searchers should always refrain from producing any written interpretation of the results that they find. All interpretation should be left to the patent attorney, and a discussion of the search deliverables should always take place before the search is complete.


To understand the common best practices applied to the other major types of short patent searching (excluding analysis projects), users should view the General Searching article and the Infringement article.

For subject-specific searching advice, see the Best Practices directory page.


  1. Hunt, David, Long Nguyen, and Matthew Rodgers, (ed.). Patent Searching: Tools & Techniques. Hoboken, NJ: Wiley. 2007.

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