On June 30, 2025, the Federal Circuit issued a precedential opinion in Eye Therapies v. Slayback Pharma in which the court interpreted the transition phrase “consisting essentially of” to be a closed term excluding other active ingredients from the claims. This decision reinforces the reality that courts, when faced with claims containing the transition phrase “consisting essentially of,” likely will include this phrase in the claim construction process and provide a meaning for it, employing usual claim construction rules to do so.

Background

There are three transition phrases: (1) comprising, (2) consisting of, and (3) consisting essentially of. “Comprising” is an open-ended term allowing for additional steps or components. “Consisting of” is a closed term excluding additional steps or components.  In AK Steel Corp. v. Sollac, the Federal Circuit held that “Consisting essentially of” permits steps or components not listed in the claims, “provided that they do not ‘materially affect the basic and novel properties of the invention.’” 

Claim interpretation analysis focuses on intrinsic evidence associated with a claim: claim language, specification, and prosecution history.

How does the United States Patent and Trademark Office (USPTO) interpret “consisting essentially of” during prosecution? As noted in the Manual of Patent Examining Procedure, “consisting essentially of” will be construed as equivalent to “comprising” for purposes of searching for, and applying, prior art absent a clear indication in the specification or claims of what the basic and novel characteristics are.  

Finally, as the Federal Circuit noted in Eye Therapies, a patentee can alter the typical meaning of “consisting essentially of”: “for example, the patentee can make the intended meaning clear in the specification or disclaim an alternative meaning in the prosecution history. (citing the Court’s 2009 decision in  Ecolab, Inc. v. FMC Corp.)

Interpreting “Consisting Essentially Of”

When a claim having the transition phrase “consisting essentially of” is involved in litigation, the court likely will determine the meaning of this phrase during claim construction (Markman) procedures based on the available intrinsic evidence. A primary focus of the court’s inquiry should be determining the basic and novel properties of the claimed invention -- such properties lie at the heart of the scope and meaning of this transition phrase, and thus the claim, as these properties are critical to determining whether any step or component is excluded from the claim because it has a material effect on the properties. Stated another way, no one can determine whether a material effect exists to exclude steps or components from claims without knowing which properties to examine for the material effect.

Accordingly, the court likely will examine other claim language, the disclosure in the specification, and statements made in the prosecution history to determine the meaning of “consisting essentially of.” In Eye Therapies, the Federal Circuit focused on the prosecution history, where the patentee made definitional use of  “i.e.” to equate its claimed “methods consisting essentially of administering brimonidine” with “methods which do not include administering other active agents.” (For example, the patentee asserted: “Moreover, [the prior art reference] teaches away from methods consisting essentially of administering brimonidine (i.e., methods which do not include administering other active agents).”)

Key Takeaways

First, “consisting essentially of” likely will be construed during litigation, including what the basic and novel properties of the claimed invention are.

Second, the prosecution history can be important in determining the meaning of “consisting essentially of” as it was in Eye Therapies. However, the other claim language and the disclosure in the specification should not be forgotten. These other pieces of intrinsic evidence can be important as well. (As the Federal Circuit wrote in Eye Therapies, “for example, the patentee can make the intended meaning clear in the specification...” And in the Manual of Patent Examining Proceedings, the USPTO construes “consisting essentially of” as equivalent to “comprising” absent a clear indication in the specification or claims of what the basic and novel characteristics are.”

Third, patent applicants should be careful when using “i.e.” in either the specification or prosecution history. A court could very well interpret this abbreviation as definitional, thereby limiting claim scope to what follows it.