Doctrine of Equivalents


Doctrine of Equivalents

In order to prove patent infringement, a patent holder must demonstrate that the alleged infringing item has sufficiently copied the original invention. Because, historically, it was rare for a second invention to consist of literal infringement, or an exact, identical replica of the original patented item, patent law recognizes the doctrine of equivalents.

This doctrine gives plaintiffs greater ability to succeed in patent infringement actions if claimed elements are sufficiently equivalent in both the original and the copy. If any claims are not present in the second invention, the patent holder may not be successful in an infringement action.

In recent years, however, literal infringement has been on the rise, while doctrine of equivalents claims have decreased.  Regardless, the doctrine of equivalents remains an important tool for patent holders to utilize when enforcing their intellectual property rights.

Doctrine of Equivalents Patent

Although in the United States, patent law is largely controlled by federal statutes, the doctrine of equivalents is a common law right, as it stems from a number of Supreme Court cases.

This doctrine stems from the public policy that an infringer should not benefit from an invention that it obviously copied, while only changing minor, superficial details on the claimed patent.

The doctrine of equivalents applies when:

  • The two elements are interchangeable, and
  • A person with ordinary skill in the art would have known the two elements are interchangeable at the time of the infringement.

In determining whether the doctrine of equivalents is applicable in a particular infringement case, courts will compare the two items to determine how close the allegedly infringing item adheres to the claimed elements of the original patent.

Courts will also examine the history of the second invention, as well as the thought process that went into its design. Documents and communications revealed during the discovery process may reveal evidence of intentional copying, or attempts to design around the existing patent, which could affect the outcome of the case. However, it is not necessary to show the defendant intentionally infringed upon the plaintiff’s patent rights.

The doctrine of equivalents can apply to technological equivalents developed after a patent is approved.

The plaintiff has the initial burden of showing that the elements of infringement are present. Once the plaintiff has met its burden, the defendant has an opportunity to raise a number of affirmative defenses in its favor. A common theme among affirmative defenses raised in the face of doctrine equivalents claims is the public policy that patent holders may only enforce their intellectual property rights to the extent of their patent’s grant.

While judges are responsible for claim interpretation, determining whether equivalence exists is a question of fact for a jury to decide. To succeed in its case, the plaintiff must meet its burden of proof by a preponderance of the evidence.

Doctrine of Equivalents All Elements Rule

In 1997, the Supreme Court adopted the “all elements rule” in Warner-Jenkinson v. Hilton Davis Chemical Co. as an addition to analysis of doctrine of equivalents cases.

This rule requires the doctrine of equivalents to apply to each individual element of a claim, not the invention as a whole. After the adaptation of this rule, it became necessary for a plaintiff to analyze the role played by each element in the context of the specific patent claim.

This rule substantially limited the extent that patent owners could bring infringement cases, but was instituted in order to prevent patent claims from growing to encompass more than was granted in the original patent approval.

Under the all elements rule, courts conducting an infringement analysis will now look at, or “read on” each patent claim against the allegedly infringing item. Both parties often bring in expert witnesses to present crucial information regarding the characteristics of the invention.

If an element is missing in the allegedly infringing object, equivalency will only be considered if the missing element amounts to a superficial difference.

Triple Identity Test

One way to determine whether differences are insubstantial is to apply the triple identity test, also known as the “function, way, result,” test.

In Graver Tank & Mfg. Co. v. Linde Air Prods., Inc., the Court established this analysis. A product is equivalent to another if:

  • It performs the same function as the original,
  • It performs that same function in the same way, and
  • It achieves the same result as the limitation that is literally recited in the patent claim.

While the triple identity test is not the only test used to analyze whether infringement exists, it is one of the oldest tests utilized in doctrine of equivalents cases. Courts often look to this test in cases involving mechanical or electrical patents, as their claims can easily be broken down into the function, way, result framework.

Insubstantial Difference Test

Under the insubstantial difference test, courts ask whether the accused product or process is substantially different from what is patented, again looking at all elements claimed in the patent application. Substantial changes between the two items may clear the defendant from infringement claims, while insubstantial or superficial changes will not.

While guidance under the insubstantial differences test is less clear than the rule set out the triple identity test, it can be more useful in analyzing patents issued for chemical arts, as well as business processes and methods.

Statutory Equivalents and “Means or Step Plus Function” Limitations

Although the doctrine of equivalents is primarily found in the common law through court decisions, there is a codified statutory equivalents rule in  35 USC 112(f) (§112 ¶6).  It was introduced to Congress as a narrow response to a 1946 Supreme Court case, Halliburton Oil Well Cementing Co. v. Walker, which otherwise would have prevented raising any equivalents claims in functional claims. This paragraph only applies to patents drafted under the “means or step plus function” approach, which, under current jurisprudence, cannot rely on the doctrine of equivalents when bringing an infringement claim.

Unlike the common law, or judicial, doctrine of equivalents rule, which in some ways expands the scope of patent rights protection, the rule of statutory equivalents has the effect of narrowing the scope of protection given by a patent.

Moreover, the statutory equivalents rule only applies to technology and equivalents available when the patent was issued, where under the common law doctrine of equivalents, courts may consider technological developments and understanding after patent is granted.

Determining whether §112 ¶6 applies to patent claims is a question of claim construction, which is a question of law reviewed by judges in a Markman hearing.  Claims brought under both the common law doctrine of equivalents and the statutory equivalents rules are subject to the same affirmative defenses, including claims of prosecution history estoppel.

Defenses to Infringement Claims Based on the Doctrine of Equivalents

In patent litigation cases in which plaintiffs base their arguments on the doctrine of equivalents, defendants have a number of affirmative defenses available.

A common theme among these defenses is that a plaintiff attempting to bring an infringement claim based on the doctrine of equivalents cannot claim more rights in their patented invention than were approved by the U.S. Patent Office in the original patent grant. These defenses reflect the public policy that it should be clear what is and is not an infringing action or item, as well as the need for patent applicants to be clear and distinct when drafting their claims.

Prosecution History Estoppel and the Presumptive Bar Approach

Often, defendants in patent infringement cases are able to avoid infringement claims by arguing prosecution history estoppel, also known as file-wrapper estoppel.

To argue estoppel, a defendant must show:

  • The plaintiff made a statement, that
  • The defendant reasonably relied on
  • The defendant is now facing legal detriment by its reliance, and
  • Justice requires enforcement of the plaintiff’s statement.

In patent infringement cases in which estoppel is raised, courts look to statements made by the plaintiff during its patent application process, known as patent prosecution.

When a patent is being prosecuted, a plaintiff may make amendments to the original patent claims, based on communications with the patent examiner. If a plaintiff amends the claims during patent prosecution, prosecution history estoppel would then prohibit a plaintiff from asserting broader claims in a later infringement case.

When raising claims of estoppel, a defendant in a patent infringement case can also point to written statements or oral arguments made during the patent prosecution that limited the scope of protection being sought.

This affirmative defense is in line with court rulings, prohibiting patent holders from expanding their patent claims from what was originally granted by the U.S. Patent Office.

While courts recognize prosecution history estoppel as a limitation to the doctrine of equivalents, they also recognize that patent holders may not have intended to narrow or surrender claims that were amended out of their patent applications.

Thus, when a defendant successfully raises prosecution history estoppel as a defense to a doctrine of equivalents claim, the court can assume that the patent does not extend to the original claims, but, under the presumptive bar approach, the plaintiff has the ability to refute this assumption.

Under this approach, the plaintiff will bear the burden of proof to show:

  • The equivalent was unforeseeable at the time the claim was drafted;
  • The amendment did not surrender the particular equivalent in question; or
  • There was some reason why the patentee could not have recited the equivalent in the claim.

In reaching a determination on prosecution history estoppel, courts will thus look at the reasoning behind the amendments before determining whether the plaintiff waived its rights to any equivalents claims. If the plaintiff’s reasoning was not to limit the patent, it may still be successful in its infringement claims.


The vitiation rule is another common defense raised in the face of a doctrine of equivalents claim. For this rule to apply, the elements of the two items in question must be so different, there is no possible way for equivalency to be established. In other words, in this situation, the legal meaning of infringement is impaired, or vitiated. Generally, in vitiation cases, the two elements at question are complete opposites from one another, or the second element is the antithesis of the claimed element.

Vitiation is brought up in summary judgment motions, when a judge may determine that no question of material fact remains for a jury to decide. When vitiation is raised, a judge will review the totality of the circumstances to determine whether the evidence would prevent a reasonable jury from determining that two elements are equivalent.

Ensnarement of Prior Art

Ensnarement is an affirmative defense to doctrine of equivalent claims that prevents patent holders from gaining rights over, or “ensnaring” prior art – inventions or knowledge that were known before the invention of the patent – that was not part of the original patent grant.

To determine whether a patent holder is attempting to ensnare prior art in an improper way, the court will analyze a “hypothetical patent claim,” where it will determine whether the Patent Office would have accepted the claim over the prior art at the time the patent was granted.

If the court believes the Patent Office would have done so, the patent holder may claim the prior art as being within the scope of its patent monopoly. If not, the patent holder’s doctrine of equivalents claims may fall short.

Dedication to the Public

Finally, another affirmative defense that defendants can raise occurs when, in the original patent application, the plaintiff disclosed subject matter in the specifications, but did not claim this subject matter. Any unclaimed subject matter listed as an alternative to a claim limitation is considered dedicated to the public and cannot be recaptured by the patent holder.

This defense illustrates the importance of patent drafting, specifically how patents must be drafted with an eye to enforcing potential infringement claims in the future.

Reverse Doctrine of Equivalents

The reverse doctrine of equivalents is a theory that could be used to avoid a finding of infringement, even if infringement technically exists.

This doctrine comes from an 1898 Supreme Court case, Westinghouse v. Boyden Power Brake Co., which laid out the two elements that must be present to find infringement:

  • First, the patent holder must show that the allegedly infringing object falls within the scope of the patent claims.
  • Second, the patent holder must show there is substantial identity between the claimed invention and the allegedly infringing invention.

The doctrine of equivalents comes into play in the first prong of this test – although there may not be literal infringement, there may be insubstantial differences in the two items that amount to infringement.

The reverse doctrine of equivalents appears in the second prong of this test. The Graver Tank Court acknowledged this doctrine, stating that an invention that falls within the literal words of a claim, but performs the same function in a substantially different way, may avoid a finding of patent infringement.

However, despite the availability of this claim as an affirmative defense in the face of patent infringement claims, as well as its regular application by district courts until the late 1980s, the Federal Circuit has never issued a decision finding noninfringement based on the reverse doctrine of equivalents.

Instead, the Federal Circuit has determined that reverse equivalency is an affirmative defense, rather than requiring plaintiffs in patent infringement cases to prove substantial identity, the second prong of the infringement test. Moreover, it has limited application of this doctrine so that it is applicable in cases involving literal infringement claims, not doctrine of equivalents claims.  As a result, while the reverse doctrine of equivalents remains an available theory, the ability to rely on it in infringement cases may be limited.

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