The U.S. Court of Appeals for the Federal Circuit recently issued its decision in Enviro Tech Chemical Services, Inc. v. Safe Foods Corp., 174 F.4th 919 (Fed. Cir. 2026), affirming a judgment that claims reciting a pH “of about 7.6 to about 10” are invalid as indefinite under 35 U.S.C. § 112(b). The Court reaffirmed that words of approximation such as “about” and “approximately” are not inherently definite or indefinite. But where none of the claims, the specification, or the prosecution history conveys how far the recited endpoints may be stretched, a skilled artisan cannot ascertain the scope of the claim with reasonable certainty.
As discussed below, the decision is less a change in the law of indefiniteness than a caution about how terms of degree are handled across the entire intrinsic record. The patentee lost not because it used the word “about,” but because its specification used inconsistent pH tolerances: in most examples experiments proceeded only within one observed deviation from a pH target, while in other examples they proceeded at a larger pH deviation. Enviro Tech’s office action responses also used the term “about” inconsistently and never explained what it meant.
Background
Enviro Tech’s U.S. Patent 10,912,321 is directed to methods of treating poultry during processing with peracetic acid, one stated goal being to increase a processing plant’s percent yield. Representative claim 1 requires providing peracetic acid-containing water in a reservoir, determining the pH of that water, and altering it “to a pH of about 7.6 to about 10 by adding an alkaline source.” This determining-and-altering sequence was performed both before and after a poultry carcass is placed into the reservoir.
Enviro Tech sued Safe Foods Corp. in the Eastern District of Arkansas. During claim construction, Safe Foods argued that two terms of claim 1, “an antimicrobial amount” and “about,” were indefinite. The district court agreed as to both and entered judgment that the asserted claims were invalid. Because the Federal Circuit affirmed as to “about,” which disposed of every asserted claim, it did not reach the alternative ground.
The Decision
Indefiniteness is reviewed de novo and the Court applied the standard from Nautilus, Inc. v. Biosig Instruments, Inc.: a claim is indefinite if its language, read in light of the specification and the prosecution history, fails to inform those skilled in the art about the scope of the claims with reasonable certainty.
The Court reiterated its long-standing rule that terms of approximation may properly be used to avoid a strict numerical boundary, and that terms of degree are not inherently definite or indefinite. What matters is whether the parameter’s range is reasonably certain based on the technological facts of the particular case. The Court considered the patent specification, prosecution history, and extrinsic evidence to determine what range was covered by the claims.
The Claim Language
Claim 1 offered no guidance on how far below a pH of 7.6, or above a pH of 10, the solution could fall and still meet the limitation. The parties agreed that the ordinary meaning of “about” is “approximately,” but the Court observed that substituting one word of approximation for another supplies no additional guidance. Even where a term of degree can be reduced to words, the claim remains indefinite if a skilled artisan cannot translate that definition into a meaningfully precise claim scope.
The Specification
The specification recited numerous experiments in which Enviro Tech set a target pH, measured the actual pH, and then decided whether to proceed. In the majority of those experiments, it proceeded only where the deviation from target was 0.3 pH units or less, and where the deviation was greater than 0.3, it would most often alter the pH to be closer to the target before proceeding. But there were exceptions. In one experiment involving 5,800,000 chickens at a major U.S. poultry processor, Enviro Tech proceeded with deviations between 0.35 and 0.5 pH units. The Court found that example particularly informative because it involved the largest number of birds and was conducted in a processing plant, which is one goal identified in the patent as the object of the invention. The specification’s conflicting practice therefore indicated that the 0.3 figure was not a firm indicator of what “about” meant.
The Prosecution History
The Court described Enviro Tech’s treatment of the term during prosecution as inconsistent. In one office action response, Enviro Tech argued that “a peracetic acid solution at the lower end of the claimed range, pH 7.6,” would not have been obvious over the prior art, thus omitting “about” from the argument entirely. On the very next page of the same response, addressing a different claim, it argued that “a step of adjusting the pH to the range of about 8 to about 9” would not have been obvious over the same prior art, this time including the qualifier. The Court concluded that Enviro Tech had treated the term as material to some claims and immaterial to others, and noted that at no point in the entire prosecution history did it explain what “about” means.
Enviro Tech argued that its narrowing of the lower boundary from “about 7.3” to “about 7.6” demonstrated a surrender of any deviation greater than 0.3 pH units and thus supplied the missing definition. The Court was not persuaded. Enviro Tech identified no repeated and consistent remarks capable of defining the term, and its amendment was not accompanied by explanatory remarks supporting the proposed construction. It had simply never offered the examiner an argument indicating what the term meant.
The Proximity of the Prior Art
The Court identified one further consideration as “an important determinant” in its decision: claim 1 had been amended with respect to pH precisely to avoid prior art disclosing a pH as close as 7.0, while the specification recited pH values from 6 to 10. Where a numerical endpoint is moved to clear art that is, in the Court’s words, “almost ‘about’ a pH of 7.6,” § 112 demands considerably more clarity than an unbounded qualifier can supply.
Guidance for Applicants
Define Terms of Approximation, or Apply a Consistent Convention
The most direct response to Enviro Tech is an express definition in the specification, e.g., that “about” means within ±0.2 pH units of the stated value, or within the measurement error of a specified method. Where an express definition is undesirable, the specification should at least apply a single, consistent tolerance across the working examples. A specification that adheres to one variance in most experiments and departs from it in others supplies evidence of ambiguity rather than evidence of flexibility.
Use Terms of Degree Consistently Within a Single Response
Every office action response is intrinsic evidence, and Enviro Tech turned in part on two arguments appearing on consecutive pages of one paper. Where a term of degree appears in the claims, arguments distinguishing the prior art should reproduce the claim language as written. If an argument depends on the precise recited endpoint rather than the qualified range, that distinction should be stated deliberately and explained, not left to be inferred from the omission of a word.
Put the Meaning on the Record When the Term Is Doing Work
The Court emphasized not only inconsistency but silence: Enviro Tech never offered the examiner any argument indicating what “about” means. Where a qualified endpoint is the basis for patentability, the response should state what the applicant understands the qualifier to encompass and identify the supporting disclosure. Doing so converts a potential ambiguity into a definitional statement. The tradeoff, of course, is that such a statement may also operate as a disclaimer, which should be weighed deliberately rather than avoided by default.
Reassess the Qualifier When Amending in View of Close Prior Art
The narrower the margin between the amended endpoint and the reference, the less tolerance the definiteness requirement will have for an open-ended qualifier. Where a range is amended to clear art sitting just outside the new boundary, consider reciting the endpoint without “about,” or reciting an express tolerance, and preserving broader coverage in a dependent claim or a continuation rather than in the qualifier itself.
Conclusion
Enviro Tech does not put terms of approximation off limits, and the Court took care to reaffirm that they remain permissible. What the decision confirms is that the intrinsic record must actually answer the question the qualifier raises. Where the specification’s own examples are inconsistent, the prosecution arguments are inconsistent, and the applicant never explains the term to the examiner, the resulting uncertainty is attributed to the patentee and, as here, can invalidate every asserted claim.
