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Patent claims using general terms of approximation such as “about” or “approximately” may be at risk of invalidation when the terms are not well-defined. In a recent patent infringement case, Enviro Tech,[1] the Federal Circuit affirmed a district court ruling that all asserted claims of U.S. Patent No. 10,912,321 were invalid as indefinite because the term “about” used in the claims did not have a clear meaning. The patent claims were directed to methods for treating poultry with peracetic acid to increase the weight of the poultry. In relevant part, the patent claimed processes for altering the pH of the treatment water to “a pH of about 7.6 to about 10.”

Why Did the Court Find “About” Indefinite in This Instance?

Under 35 U.S.C. § 112(b), a patent’s claims must “particularly point out and distinctly claim the subject matter which the inventor or a joint inventor regards as the invention.” Under this statute, courts may hold that a claim is indefinite if it fails to inform, with reasonable certainty, those skilled in the art about the scope of the invention. The Federal Circuit reiterated that terms of degree like “about” and “approximately” are not inherently definite or indefinite under 35 U.S.C. § 112(b). Rather, they must be described with reasonably certainty, which depends on the particular technological facts of each case. Here, the court examined the claims, specification, and prosecution history and found that none provided the needed clarity, but rather when considered together created ambiguity as to the meaning of the term “about” as used in the patent claims. As a result, the claims were held to be invalid.

• The Claims: The claim language offered no guidance on how much the pH could deviate below 7.6 or above 10 and still fall within the scope of “about.”
• The Specification: The specification did not define “about.” While most experiments in the specification proceeded only when pH deviated by 0.3 or less from the target, several other experiments, including the largest and most commercially representative one, proceeded with deviations of 0.35 to 0.5. These inconsistencies undermined any argument that “about” meant a defined pH tolerance of 0.3.
• The Prosecution History: Enviro Tech treated the term inconsistently during prosecution, sometimes including “about” in its arguments and sometimes omitting it, without explaining what the term meant. Critically, the claims had been amended to change the lower pH bound from “about 7.3” to “about 7.6” to distinguish over prior art at pH 7.0, making the vagueness of “about” especially problematic given how close the prior art was to the claimed range.

Key Takeaways for Patent Strategy

This decision does not prohibit the use of “about” or “approximately” in patent claims. It does, however, reinforce that patent holders should provide clarity when using such terms, else there may be an invalidation risk. If a patentee chooses to use words of approximation, a few practical steps to mitigate risk include:

1. Defining terms of approximation in the specification. If “about” means ± 0.3 units, or ± 5%, or some other tolerance, a patentee should state so explicitly. Such a definition may be applied generally to the term “about,” or with respect to the use of “about” along with a particular parameter.
2. Ensuring consistency across the specification. Conflicting examples of acceptable deviations will weaken a patentee’s position. All described embodiments, and experimental data where feasible, should reflect a uniform understanding and use of the approximation term.
3. Being deliberate during prosecution. Inconsistent treatment of an approximation term in statements made to the Patent Office can be used against a patentee. If “about” is meaningful in one claim, a patentee should treat it consistently across all claims and responses.
4. Exercising particular caution near prior art boundaries. When a claimed range is amended to avoid prior art, a term of approximation like “about” may create fatal ambiguity as to whether the prior art still falls within the scope of the claim. If there is ambiguity over the meaning of the term “about,” it may not be advisable to add or retain the term when amending the claims to distinguish from a prior art range.
5. Introducing non-approximate dependent claims. If a term of approximation is used in an independent claim, one or more dependent claims may be added which provide particular specificity to otherwise potentially indefinite ranges. For example, one dependent claim may recite the range including the intended approximation (e.g., a pH of 7.3 to 10.3) and another may recite the range without the intended approximation (e.g., a pH of 7.6 to a pH of 10).

Patent applicants and owners should exercise caution when drafting, prosecuting, or enforcing claims that rely on undefined terms of approximation, especially where those terms define boundaries that are critical to distinguishing over prior art. However, with careful attention to the definition of terms of approximation at the drafting stage as well as conscious awareness of these terms thereafter, risks can generally be mitigated.

[1] Enviro Tech Chemical Services, Inc. v. Safe Foods Corp, No. 2024-2160, slip op. (Fed. Cir. Dec. May 4, 2026). In the case, Enviro Tech sued Safe Foods alleging infringement of U.S. Patent No. 10,912,321. At the Eastern District of Arkansas, Safe Foods won its invalidity arguments on the basis of the term “about” in independent claim 1 as discussed herein, as well as on the basis that the term “an antimicrobial amount” also appearing in independent claim 1 was also indefinite. Because the CAFC affirmed invalidity on the basis of “about” being indefinite, it did not further address the alternative ground that “antimicrobial amount” was indefinite.