Federal Circuit Finds the Word ‘About’ to Be Unclear: When Terms of Degree Cause Indefiniteness

The US Court of Appeals for the Federal Circuit’s recent decision in Enviro Tech Chemical Services Inc. v. Safe Foods Corp. affirmed a finding that the term “about” rendered the claims indefinite. This precedential opinion illustrates how inconsistencies in the intrinsic evidence can cause terms of degree—which are often permissible—to run afoul of Nautilus’s requirement that claim scope be “reasonably certain” to a person of ordinary skill in the art.

BACKGROUND

Enviro Tech sued Safe Foods Corp. for allegedly infringing US Patent No. 10,912,321 (’321 Patent), which is directed to methods for treating poultry with peracetic acid to increase its weight. [1] Claim 1 requires adjusting the peracetic acid solution’s pH to “about 7.6 to about 10 by adding an alkaline source.” During claim construction, Safe Foods argued that the term “about” was indefinite. The district court agreed, finding both terms invalid. Enviro Tech then appealed to the Federal Circuit.

FEDERAL CIRCUIT’S DECISION

On May 4, 2026, the Federal Circuit issued a precedential opinion affirming the district court’s finding that “about,” as used in the claims, is indefinite. [2] The Federal Circuit acknowledged its prior holdings that terms of degree like “about” or “approximately” are not inherently indefinite. However, the court emphasized that claim scope must be “reasonably certain based on the technological facts of the particular case.” The panel proceeded to explain how the intrinsic evidence led to indefiniteness in this particular case.

Claim Language

The panel first highlighted how the claim language itself provided no guidance regarding how much deviation from the recited endpoints would fall within the scope of “about.” The claims recited adjusting the pH of the peracetic acid solution to a “pH of about 7.6 to about 10 by adding an alkaline source,” but nothing more. And although the parties agreed “about” means “approximately,” the court found that equivalence unhelpful because substituting a synonym did not clarify the permissible range of deviation.

Specification

Moving next to the specification, the Federal Circuit noted inconsistencies in how target pH levels were set in the disclosed examples. Those examples involved experiments where the actual pH was adjusted to within 0.3 units of the target pH. There were notable exceptions, however, including a large-scale experiment involving 5.8 million chickens where deviations of up to 0.5 were tolerated. As the panel explained, this suggested an inconsistency in what “about” practically encompassed. The panel concluded such conflicting guidance would not allow a skilled artisan to determine the boundaries of the claimed range with reasonable certainty.

Prosecution History

Turning last to the prosecution history, the panel found nothing further that clarified the scope of “about.” During prosecution, Enviro Tech treated “about” inconsistently and never clarified its meaning to the examiner. For example, Enviro Tech omitted “about” when discussing the lower pH boundary in certain arguments but included the term in other arguments. The Federal Circuit acknowledged the claim had been amended from “about 7.3” to “about 7.6” during prosecution but found no supporting remarks or consistent explanation in the record that would have suggested this amendment limited the permissible variance to 0.3, rather simply altering the numerical endpoint itself.

The Federal Circuit concluded by noting that “[a]n important determinant in our decision here is the very fact that what is now claim 1 was amended in respect to pH to avoid prior art, which was as close as a pH of 7.0.”

KEY TAKEAWAYS

The Federal Circuit’s holding reflects a somewhat more stringent application of the Nautilus standard to a term of degree. [3] An “important determinant” underlying that application was the fact that a prior art reference fell squarely within the area of alleged ambiguity.

For patent challengers, this holding reinforces the persuasiveness of exemplary evidence in an indefiniteness challenge. Rather than simply identifying potential ambiguity, it is advantageous to provide courts with a concrete example (here, prior art with a pH of 7.0) of how that ambiguity results in “reasonable uncertainty” about the claim’s scope.

For patentees, the holding reemphasizes the importance of consistency across the intrinsic evidence:

• Inconsistency across the intrinsic record can sink claims: Conflicting disclosures in the specification or inconsistent use during prosecution can undermine definiteness, even for otherwise permissible terms of degree.
• Drafting should reflect disclosed tolerances: If experimental data or real-world practices reflect a range of acceptable deviations, that range should be clearly and consistently articulated in the specification.
• Silence during prosecution can be costly: Failure to explain the meaning of key claim terms may produce indefiniteness vulnerabilities.