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File History Estoppel and the Doctrine of Equivalents

Posted by Isabel Fox | Oct 11, 2021 | 0 Comments

What is File History Estoppel and How Does It Relate To the Doctrine of Equivalents?

File history estoppel, also known as file wrapper or prosecution history estoppel, is, like the doctrine of equivalents, an equitable doctrine developed over the years by the federal judiciary in its consideration of patent infringement cases. At its most basic, file history estoppel means that amendments or statements made by the patent applicant during the prosecution of a patent application can preclude a patent owner from arguing an alleged infringing product infringes its patent either directly or under the doctrine of equivalents. This preclusion is known as an estoppel.

The reasoning underlying file history estoppel is that a patent owner should not be able to reclaim in an infringement suit that patent scope which was surrendered by argument or amendment to get their patent issued – essentially, once surrendered, always surrendered.

For example, patent applicants may make arguments regarding a prior art reference in an attempt to distinguish their invention(s) over the cited references. If these arguments are accepted by the United States Patent Office such that a patent issues, there is a strong, if not most likely, chance that the patent applicants will be barred from arguing infringement counter to those arguments. Similarly, any claim amendments made to distinguish pending claims over the prior art will prevent the patent owner from claiming infringement on a basis that runs counter to those amendments. Further, amendments made to satisfy the requirements of the Patent Act may also give rise to an estoppel.

In Warner-Jenkinson (see “Evolution of the Doctrine of Equivalents through Case Law,” posted September 30, 2021) an argument was made that infringement under the doctrine of equivalents was impermissible. As noted in the previous article, during prosecution of their patent directed to a process for dye production, Hilton Davis Chemical Co. amended the claims to recite a pH of 6.0 to 9.0 while Warner-Jenkinson Co. practiced their process at a pH of 5.0. Hilton Davis sued Warner-Jenkinson for infringement under the doctrine of equivalents, and Warner-Jenkinson argued that infringement under the doctrine of equivalents should be impermissible. The Supreme Court rejected Warner-Jenkinson's argument because, on the record before the Court, there was no indication of the reason why the lower pH limit of 6.0 was chosen by Hilton Davis during prosecution of their patent application. It was not based on any prior art of record. In such a situation, the Court stated that “the better rule is to place the burden on the patent holder to establish the reason for an amendment required during patent prosecution.”[i]

Any amendments or statements made during prosecution do not rise to an estoppel in all situations. In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., Ltd., et al.,[ii] the Court held that any claim amendments made during patent prosecution must be examined in the context in which they were made before determining whether an estoppel arises that prevents a patent owner from claiming equivalency of a claim limitation. During prosecution of its patent, Festo made certain claim amendments to comply with the requirements of how to claim pursuant to the Patent Act. The Court of Appeals for the Federal Circuit held that these claims constituted a complete bar for asserting equivalency. The Supreme Court reversed this holding.

In summary, while the doctrine of equivalents is available to show infringement, prosecution estoppel might bar such assertion as it applies to both direct infringement and infringement under the doctrine of equivalents.


The Doctrine of Equivalents provides patent owners with another route to protect their Intellectual Property, either in Federal Court patent infringement actions or in International Trade Commission exclusion procedures. Whether a patent owner or someone accused of infringement, our attorneys can help you by investigating whether patent rights are being infringed either directly or pursuant to the doctrine of equivalents. In either situation you are encouraged to seek legal advice promptly.  Please call us at 651-256-9480 or fill out this Contact Form to speak with us before it's too late.

*The information provided in this article is not legal advice and should not be relied on as such. The content of this article is for informational purposes only and is meant as a starting point in your search for answers to your legal questions.

[i] 520 U.S. at 33.

[ii] 535 U.S. 722 (2002)

About the Author

Isabel Fox

Isabel Fox is a registered patent agent and has been with Gallium Law since 2018. Her practice largely revolves around utility and design patent prosecution; including conducting patent landscape searches, drafting patent applications, and responding to Office Actions issued by the USPTO...


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