Recent decisions across the Federal Circuit and district courts reflect a tightening approach to approximation in patent claims. In Enviro Tech Chemical Services, Inc. v. Safe Foods Corp., the Federal Circuit invalidated claims where “about” lacked objective boundaries. Enviro Tech Chem. Servs., Inc. v. Safe Foods Corp., No. 2024-2160, 2026 WL 1204374 (Fed. Cir. May 4, 2026). In Arbutus Biopharma Corp. v. Moderna, Inc., removal of “about” during prosecution triggered estoppel and foreclosed reliance on the doctrine of equivalents. Arbutus Biopharma Corp. v. Moderna, Inc., No. 1:22-cv-00252-JDW, 2026 WL 266389 (D. Del. Feb. 2, 2026). And in Fennec Pharmaceuticals Inc. v. Cipla Ltd., even an expressly defined “about” required expert interpretation at claim construction. No. 2:23-cv-00123 (D.N.J. July 14, 2025). The unifying takeaway is not necessarily that “about” is per se improper. Instead, these decisions collectively demonstrate that use of “about” may create downstream risk and should be carefully considered with advice from patent prosecution counsel.
Enviro Tech: Indefiniteness Where “About” Lacks Coherent Boundaries
In Enviro Tech, the Federal Circuit affirmed invalidity of claims reciting a pH “of about 7.6 to about 10.” The court reiterated that terms of degree are permissible only where the intrinsic record provides a reasonably certain boundary, and that the applicant’s “treatment of the word ‘about’ during the patent’s prosecution … fails to inform a skilled artisan of claim 1’s scope with reasonable certainty.” Enviro Tech Chem. Servs., Inc. v. Safe Foods Corp., No. 2024-2160, 2026 WL 1204374, at *3 (Fed. Cir. May 4, 2026). The court noted that claims at issue were silent as to tolerance. Id. (“[T]he claims do not provide any guidance on how much below a pH of 7.6 or above a pH of 10 the peracetic acid-containing water can be to meet the limitation.”). The specification provided conflicting guidance, with some examples suggesting ±0.3 pH units and others tolerating deviations up to 0.5 pH units. Id. at 5. The prosecution history offered no clarification; indeed, “Enviro Tech treated the term inconsistently, suggesting that it was material to some claims and immaterial to others.” Id.
Against a backdrop of prior art at pH 7.0, the court required precision that the term “about” did not supply. The court concluded that “[w]hen the specification recites pHs of 6–10, and the prior art pH of 7.0 required the claims to be amended, the definiteness requirement of § 112 necessitates much more clarity than using the vague term ‘about.’” Id. (internal citations omitted). Enviro Tech may reflect increased scrutiny where “about” is used in close proximity to prior art without a clear and consistent definition.
Arbutus: Removing “About” and the Resulting Prosecution History Estoppel
While Enviro Tech reflects the risks associated with using “about” without sufficient definitional support, the recent rulings in Arbutus Biopharma Corp. v. Moderna, Inc. illustrate the consequences of eliminating the term during prosecution. In advance of trial, the District of Delaware held that Arbutus’ amendment during prosecution, which removed “about” from claimed lipid molar ratio ranges, constituted a narrowing amendment that gave rise to prosecution history estoppel and precluded the patentee from asserting infringement under the doctrine of equivalents (DOE). Arbutus Biopharma Corp. v. Moderna, Inc., No. 1:22-CV-00252-JDW, 2026 WL 266389, at *8-10 (D. Del. Feb. 2, 2026). As a result, Arbutus was limited to proving literal infringement of the asserted claims. Id. at *1.
The practical effect of that ruling is significant. The asserted patents claim lipid nanoparticle formulations defined by specific molar percentage ranges for multiple lipid components. Because the patentee was limited to literal infringement, even minor deviations in component ratios that fall outside the literal scope of the claims would be held not to infringe. From a drafting and prosecution perspective, Arbutus illustrates that removal of “about” may have consequences beyond simple clarification. Indeed, the court found the applicant’s assertion that the amendment was made “[i]n an earnest effort to expedite prosecution, but without acquiescing on the merits of the rejection” insufficient to obviate the estoppel. See id. at *3. Accordingly, while such amendments may be an expedient way to resolve an Examiner’s indefiniteness concerns or distinguish prior art, they can also operate as a surrender of any tolerance that might otherwise have been captured through equivalence. In technologies where small numerical variations are expected – including pharmaceutical formulations and biologics – such surrender may be dispositive.
Fennec: Even Defined “About” Can Introduce Interpretive Instability
In the District of New Jersey, the claim construction decision in Fennec demonstrates that defining “about” may not resolve all interpretive questions. A pair of pH terms were at issue in Fennec: “a pH between about 6.5 and about 8.9” (’793 Pat.) and “a pH between about 8.6 and 8.8” (’793 Pat.). In each patent, the word “about” is defined as “any values, including both integers and fractional components that are within a variation of up to ±10% of the value modified by the term ‘about,’” though the court noted that neither patent defined “value.” Fennec Pharmaceuticals Inc. v. Cipla Ltd., No. 2:23-cv-00123 at 4-5 (D.N.J. July 14, 2025). The case settled shortly thereafter.1
Critically, because pH is measured on a logarithmic scale, the court had to determine whether the ±10% variation applied to the numerical pH value or to the underlying ion concentration. Plaintiff asked the Court to construe the values presented in the claims as logarithmic measurements of the concentrations of hydronium ions (H+) and hydroxide ions (OH–), arguing that, due to the logarithmic nature of the pH scale, a 10% difference in the number of the pH scale measurement does not necessarily represent a 10% difference in the underlying value. Plaintiff accordingly proposed a construction in which the word “about” modifies not simply the number of the pH measurement, but rather the underlying concentration of ions (resulting in a construed pH range of between 8.56 and 8.85). Defendants, in turn, argued that the value modified by the word “about” is simply the face value of the numbers of the pH measurement, which yielded a much broader proposed pH range than Plaintiff’s (e.g., between 7.74 and 9.68 (8.6 – 0.86 = 7.74 and 8.8 + 0.88 = 9.68). Id. at 5 Relying on expert testimony, the court adopted the Plaintiffs’ approach. Specifically, the court found that “given the patents’ silence on the meaning of the word ‘value’ and the importance of that word as it relates to the modifier ‘about’ in the context of the pH scale,” the court needed to rely on extrinsic evidence. Id.
Finding Plaintiff’s expert persuasive, the court found that “the ‘value’ being measured by the pH scale is acidity/basicity, which in turn is a measure of the concentration of H+ and OH- ions” and that “[a]s such, the Court finds that Plaintiff’s construction of the pH claims more accurately describes the way a person of ordinary skill in the art would interpret the meaning of ‘about’ as it pertains to pH values.” Id. at 5-6. The court narrowly construed the “about” term modifying the pH range and adopted Plaintiff’s proposed narrow construction. Id. at 6. This result is notable for its finding that, even with an explicit definition, “about” required technical reinterpretation and extrinsic evidence at claim construction, producing a narrower range than might be assumed from the claim language in view of the specification definition alone.
Synthesis and Practical Implications
Taken together, Enviro Tech, Arbutus, and Fennec highlight relevant considerations for practitioners confronting numerical claim limitations:
- Use of “about” without consistent and technically grounded support risks indefiniteness or an overbroad construction;
- Inclusion of an explicit definition in the specification may preserve validity, but can introduce interpretive complexity and potentially narrow or broaden the claims in ways not immediately apparent from the claim language;
- Removal of “about” during prosecution may foreclose reliance on the doctrine of equivalents and limit enforcement to strict literal scope.
These decisions may suggest that courts expect flexibility introduced by terms such as “about” to be supported by the intrinsic record, while also giving weight to amendments that remove such flexibility during prosecution. For patent attorneys, particularly in fields involving tightly constrained numerical parameters such as biotech and pharma, these cases highlight the importance of considering the relationship between claim language, specification support, and prosecution strategy.