Claim construction is a question of law when based on only intrinsic evidence (i.e., the patent claims, specification, and prosecution history), and therefore subject to de novo review. See MasterMine Software, Inc. v. Microsoft Corp., 874 F.3d 1307, 1310 (Fed. Cir. 2017). If extrinsic evidence is relied upon to resolve factual disputes, such as the background science or the meaning of a term to a skilled artisan, however, then a review of that factual determination is under the clear error standard—but the ultimate interpretation of the claim in light of those facts as found remains a conclusion of law subject to de novo review on appeal. Id.

Significant Figures

The claim construction issue in Actelion Pharmaceuticals Ltd v. Mylan Pharmaceuticals Inc., No. 2022-1889, — F.3d — (Fed. Cir. Nov. 6, 2023), was whether the claim language, “a pH of 13 or higher,” included a margin of error which could involve rounding up to a pH of 13.

Although extrinsic evidence from textbooks also was submitted, the district court construed the claim term as including rounding to the ones place, noting that “a numerical value includes rounding based on the inventor's selection of significant figures in the claims where the intrinsic record does not indicate otherwise.”

Intrinsic Evidence

The Federal Circuit found that the intrinsic evidence was equivocal.

The claim language did not use approximation language like “about”—but it might not be practically possible to measure exact pH values because that would require counting every hydrogen ion in solution, which is not scientifically possible.

In other words, we find both views equally plausible here; that the absence of approximation language might suggest no approximation, but that the nature of measuring a pH value might nonetheless reasonably require a margin of error.

Next, the specification explains that “[t]he pH of the bulk solution is preferably adjusted to about 12.5-13.5, most preferably 13.” The specification also used both “13” and “13.0”—and various degrees of precision for pH values generally—throughout.

Said otherwise, the specification supplies the same clarity as to the desired level of precision as muddied water.

As for the prosecution history, while the Examiner drew a distinction between the stability of a composition with a pH of 13 and that of 12; but such a distinction did not illuminate the narrower issue of whether a pH of 13 could encompass values that round to 13, such as 12.5.

Extrinsic Evidence

The Federal Circuit found “that this case is one where the proper claim construction cannot be reached without the aid of extrinsic evidence, and that the district court should have considered, at minimum, the textbook excerpts offered and addressed by the parties.”

If a disputed claim term remains ambiguous after analysis of the intrinsic evidence, then reliance on extrinsic evidence is necessary and subsidiary factual findings about that extrinsic evidence become the evidentiary underpinnings of claim construction. Teva Pharms. USA, Inc. v. Sandoz, Inc., 574 U.S. 318, 332, 135 S.Ct 831 (2015); Pickholtz v. Rainbow Techs., Inc., 284 F.3d 1365, 1372–73 (Fed. Cir. 2002); Vitronics Corp. v. Conceptronic, Inc., 90 F.3d 1576, 1583 (Fed. Cir. 1996).

Accordingly, the Federal Circuit remanded the case to consider extrinsic evidence regarding whether a pH of 13 carries any meaning to a person of ordinary skill in the art as regards precision of measurement, significant digits, or rounding.

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