Key Takeaways

  • The doctrine of equivalents allows patent holders to enforce rights even when an accused product doesn’t literally infringe but performs substantially the same function in the same way for the same result.
  • U.S. law applies the triple identity test and the all-elements rule, while defenses like the vitiation rule and file wrapper estoppel limit its scope.
  • Hypothetical claims are used to balance DOE against prior art and prevent overbroad enforcement.
  • The reverse doctrine of equivalents can protect defendants when an accused device achieves the same result but in a fundamentally different way.
  • Other jurisdictions, such as the UK (Actavis v. Eli Lilly) and Europe, apply their own forms of the doctrine, broadening the relevance of DOE globally.
  • DOE ensures patents remain enforceable against trivial design-arounds but requires careful, fact-specific analysis and often expert testimony.

What Is the Doctrine of Equivalents?

The doctrine of equivalents is a legal method for a patent owner to file an infringement claim even though the original product is not completely identical to the infringing product. 

There are a variety of ways for patent holders to protect their intellectual property. One of these methods is called the "doctrine of equivalents" (DOE). The doctrine of equivalents lets a patent holder sue for infringement if the product in question isn't exactly the same as the original. This prevents someone from making minor changes to a product in order to avoid an infringement case. 

In these cases, the Court will examine whether the infringing product is for the same purpose or completes the same function as the original product. If the product is found to be equivalent in these ways, then the patent holder will win their claim. 

This doctrine was established in Graver Tank & Mfg. Co. v. Linde Air Prods., Inc. The decision of this case resulted in the "triple identity" test. Under this test, a product is equivalent if it meets the following criteria:

  • It performs the same function as the original.
  • It performs that function in the same way.
  • It achieves the same result as the original.

A later Supreme Court case — Warner-Jenkinson v. Hilton Davis Chemical Co. — established the "all elements" rule. This rule states that each claim element must be examined instead of the whole invention. Every element of the original invention must have an equivalent in the infringing invention for a claim to stand. 

The elements of the patented invention must have substantial equivalents in the new invention. This means they meet the rules of the triple identity test. If an element is missing, it will only be equivalent if there are superficial differences in the infringing device. 

Equivalency should only be examined after a claim has been filed. It should not occur when an invention is patented. New technology developed after patenting may change the nature of essential elements. 

Expanding Infringement

In the Warner-Jenkinson decision, the Court greatly narrowed what could be considered infringement. In the past, infringement was determined by the patent. After this decision, infringement cases are more restricted. Each element of an invention must be examined. No longer is an invention examined as a whole. 

An example of this is the Brilliant Instruments v. GuideTech case. In this case, a former GuideTech worker started his own company. GuideTech sued him for infringement on one of their products. The initial ruling went in the employee's favor. However, the Supreme Court decided that the DOE applied in this case and that the employee was guilty of infringement. 

In the case, the employee redesigned a circuit commonly used by GuideTech. While the design was different, it functioned in the same way. This constituted infringement. If an invention works the same way, even if it is not designed the same, infringement has usually happened. 

Vitiation Rule

In the DOE, there is something known as the vitiation rule. This rule is a common defense against a DOE claim used when two elementsare complete opposites. Vitiation prevents the DOE from being used to make a claim term meaningless. Vitiation applies when no reasonable jury could find two elements to be equivalent. In this case, the judge would make a complete or partial decision without sending it to a jury.

It doesn't matter if two elements are designed differently so long as the work the same way, as in the GuideTech case. For vitiation to apply, elements must be so different that there is no way equivalency could be established. If a judge cannot determine equivalency, the question will be left to the jury. 

Vitiation claims are seeing something of a comeback. The reason for this is that the Federal Circuit has placed the burden on patent holders. In the Akzo Nobel Coatings, Inc. v. Dow Chemical Company case, the court examined two methods for handling dispersion in pipes. Akzo, the patent holder, lost his initial claim and appealed. The Federal Circuit found that Akzo did not prove that his invention functioned in substantially the same way as the defendant's. 

This decision shows that vitiation may be a legitimate defense. If the patent holder cannot prove that his device functions in an equivalent way to the defendant's, there is no infringement. 

Hypothetical Claim

One way for a court to determine DOE is using a hypothetical claim. A hypothetical claim determines whether a product has infringed on an existing patent by copying its prior art in such a way that consumers would assume it served the same function. An example of this can be found in Intendis GMBH v. Glenmark Pharmaceuticals Inc., USA, Glenmark Pharms., Inc.

Bayer made a gel that treated a skin condition. The gel was covered by the '070 patent. Glennmark made a generic gel and claimed the patent didn't apply. This claim was based on the fact that the generic gel contained different inactive ingredients. 

Another item in question was the lecithin content. Glennmark claimed their gel contained no lecithin. The court ruled that this could not be proven. Therefore, Glennmark's product contained the same ingredients as Bayer's. 

The court found that DOE applies even if the patent does not spell out function, way, and result. Because the Glennmark gel performed the same function as the Bayer gel and their ingredients were not substantially different, it was ruled that DOE applied.

This case shows that DOE applies even if a patent is limited in its descriptions. Making minor changes to a product does not invalidate patents. DOE still applies. 

Generally, there are three limitations on the doctrine of equivalents:

  1. The difference in the two inventions must be minor.
  2. Patent holders are limited by the claims in their patent.
  3. Equivalents cannot be used to encompass prior art.

Hypothetical claims are used to address the third limit. These claims examine whether an equivalent claimed by the patent holder would have been granted in their original patent. If not, it cannot be used in a DOE case. This serves three purposes. First, it prevents the defendant from dealing with an undue burden. Second, it protects inventions in the public domain. Third, it preserves the power of the Patent and Trademark Office to govern prior art. 

The decision in Sporting Goods v. David Geoffrey & Assoc. established hypothetical claims. This process was refined in Ultra-Tex Surfaces, Inc. v. Hill Bros. Chem. Co. and Streamfeeder, LLC v. Sure-Feed Systems, Inc. 

The question in the Ultra-Tex case involved a process for creating concrete surfaces. Ultra-Tex claimed its process, which used a polycoat paper was infringed upon by the Hill Bros. process, which used heavy wax impregnated paper. To prove infringement, Ultra-Tex used a hypothetical claim. However, in the claim, they both broadened the hypothetical claim so that it would cover the product in question and added limitations to avoid prior art.

The court decided it was improper to broaden and narrow the claim for a hypothetical. The court removed the limitations and focused on the broadened claim. Because that claim would cover prior art as well as the product in question, their claim of infringement was denied. 

The Streamfeeder decision imposed procedural limits. The court found that the burden of proving the hypothetical claim rests with the patent holder. However, once the hypothetical claim has been allowed, the burden shifts to the defendant. The defendant must show that the claim would not be granted based on prior art.  

Amending Claims

It's common for a patent to be issued with different claims than the ones with which it was originally filed. Patents can be rejected multiple times. Claims must be adjusted to be both narrower and to provide more descriptive details. If a patent is denied on the basis of prior art and the applicant adjusts their claims to avoid this prior art, they cannot make broader claims. They gave up the broader claim to allow the narrower claims to be approved. This means they cannot state the narrow claims are the equivalent of the broad claim. 

The history of your claims being rejected and amended is recorded in a file maintained by the USPTO. This history will be stored in a large paper jacket that is called a "file wrapper." Because of this, the process of a claim being rejected and then being narrowed and refilled with a limiting amendment is called a "file wrapper estoppel."

Reverse Doctrine of Equivalents

While the doctrine of equivalents expands protection for patent holders, courts also recognize a counterbalance known as the reverse doctrine of equivalents. Under this principle, an accused product may avoid infringement if, despite literally falling within a patent’s claims, it operates in a way so different from the patented invention that it cannot fairly be considered an equivalent.

For example, a technology that produces the same end result but uses a fundamentally different scientific principle or structure may be shielded under this doctrine. Courts apply it sparingly, but it illustrates how patent law seeks to strike a balance between protecting inventors and preventing overreach.

Why Doctrine of Equivalents Is Necessary

Patents usually include the following parts:

  • Drawings of the invention
  • A specification
  • One or more claims

In an infringement case, the only part of the patent that matters is the claims. To determine infringement, each claim of the patent will be read and compared to the accused product. In most cases, infringement is clear. However, sometimes this is not the case. Sometimes, a person will incorporate differences into their invention to avoid infringement. These small differences can make infringement unclear, which is the purpose of the DOE. 

The reason that DOE is such an effective tool for protecting patents is that it compares inventions in fine detail. DOE examines how an invention works, not how it looks. This allows the court to see past superficial changes to see if infringement is present. 

While the concept may seem simple, applying it is often very complex. For example, both parties in a DOE case will often have an expert witness. This can confuse the fact of equivalency. Also, an element that is equivalent for one function may not apply to another function. This can make it very hard for a patent owner to prove their case. 

Another difficulty arises from how a patent is granted. Sometimes, a person applying for a patent may need to narrow their claims for approval. This narrowing prevents them from using broader claims in a DOE case. However, if the patent holder is a pioneer in their field, they will be able to argue much broader claims. 

There is also something known as the "reverse doctrine of equivalents." In a reverse DOE case, the defendant's invention may have been altered so much that it completes the same function but in a completely different way. If this is a case, it would be almost impossible for the patent holder to prove infringement. 

If you're considering a DOE case, it's a good idea to get advice from a patent attorney. 

International Perspectives on DOE

The doctrine of equivalents is not unique to the United States. Other jurisdictions have adopted similar approaches, though often with different tests and boundaries.

  • United Kingdom (Actavis v. Eli Lilly, 2017): The UK Supreme Court redefined infringement analysis by introducing its own version of the doctrine, holding that an element not explicitly mentioned in the claims can still infringe if it achieves substantially the same result in a way the skilled person would regard as immaterial.
  • Europe: The European Patent Convention (EPC) does not expressly provide for DOE, but national courts have applied equivalents in line with the Protocol on Interpretation of Article 69 EPC, focusing on fairness and balancing literal claim wording with inventive purpose.
  • Japan and Other Jurisdictions: Japanese courts also apply a five-part test for equivalents, emphasizing whether differences are essential, obvious at the time of filing, or covered by prior art.

These international developments highlight how DOE is a globally recognized tool to prevent unfair circumvention of patents while ensuring innovation remains protected.

Full Analysis

In many cases, there is no infringement found because there has been no full DOE analysis. This was the reason the EPOS Technologies Ltd. V. Pegasus Technologies, Ltd. case was overturned by the U.S. Court of Appeals. This case involved four patents. Summary judgment was granted to EPOS. Pegasus appealed the judgment. 

The Federal Circuit vacated the decision because the district court had not fully examined both products. The initial ruling was based on a superficial difference. It did not use the "function, way, result" test required for DOE claims. When a DOE claim is filed, the Court must perform a full analysis of both products in question. 

If a full analysis has not been performed, there is no way to determine equivalency. 

Another example of this is the Deere & Co. v. Bush Hog, LLC decision. This case involved rotary cutter decks. In the initial decision, the Court ruled that because the upper deck did not touch the lower deck in the Bush Hog cutter, it did not infringe on the Deere patent. However, this was overturned. The Federal Circuit stated the two decks engaged even though they did not touch, which infringed on the Deere design. 

In DOE cases, the results are what matters. Devices that perform the same function despite minor alterations infringe on patents. 

Practical Considerations in DOE Litigation

Doctrine of equivalents cases are often highly fact-intensive and can be difficult to predict. Courts must rely on expert testimony, technical analysis, and the nuanced interpretation of patent claims. Some key considerations include:

  • Burden of Proof: The patent holder must establish equivalence for each claim element, not just the invention as a whole.
  • Expert Testimony: Technical experts often provide conflicting evidence, making jury decisions complex and uncertain.
  • Uncertainty for Businesses: Because DOE expands beyond literal claim scope, companies designing new products may face unpredictable infringement risks.
  • Judicial Trends: Recent decisions in U.S. courts show a tendency to limit DOE to prevent overly broad enforcement, while UK and European courts have expanded its application in some cases.

Overall, DOE litigation requires careful strategy, deep technical understanding, and strong legal advocacy to navigate successfully.=

Get Help With Your Doctrine of Equivalents Claim

As you can see, doctrine of equivalents cases are extremely difficult. They require a large burden of proof and detailed knowledge. If you need help planning your doctrine of equivalents case, you need to get help from an experienced attorney like those you'll find on UpCounsel. 

Frequently Asked Questions

  1. What is the purpose of the doctrine of equivalents?
    It prevents infringers from avoiding liability by making trivial or insubstantial changes to a patented invention.
  2. How is equivalence determined?
    Courts often use the “function, way, result” test, asking whether the accused product performs substantially the same function, in the same way, to achieve the same result.
  3. Can DOE apply outside the United States?
    Yes. The UK, Europe, Japan, and other jurisdictions recognize equivalents, though the tests and scope vary.
  4. What is the reverse doctrine of equivalents?
    It’s a defense where an accused product may escape infringement if it operates so differently that treating it as an equivalent would be unfair, even if claims read on it literally.
  5. How do file wrapper estoppel and vitiation limit DOE?
    File wrapper estoppel prevents patentees from reclaiming claim scope they surrendered during prosecution, while the vitiation rule bars DOE arguments that would render a claim element meaningless.

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