Choice Of Law In Patent Appeals: The Federal Circuit’s Issue-by-Issue Framework

1. The Federal Circuit applies governing law on an issue-by-issue basis, not to the appeal as a whole: The court consistently evaluates each discrete issue—such as venue, claim construction, or evidentiary rulings—to determine whether Federal Circuit or regional circuit law applies, rather than assigning a single body of law to the entire appeal.
   
   
2. Subsidiary issues in patent appeals may be governed by regional circuit law, even when outcome-determinative: Procedural and evidentiary matters like summary judgment, Rule 702 admissibility, and JMOL are typically reviewed under regional circuit standards, but their interaction with patent-specific doctrines can still drive the ultimate result.
   
   
3. The choice-of-law determination is substantive and can shape outcomes in modern patent appeals: Because different bodies of law may govern standards of review, legal frameworks, and claim interpretation, the governing-law analysis can directly influence how arguments are framed and whether a party prevails.

It can be tempting for U.S. patent practitioners to think first and foremost in terms of Federal Circuit law, given how much of their work centers on patent issues and the Federal Circuit’s exclusive appellate jurisdiction over them. But that Court’s jurisdiction does not mean that every issue in a patent appeal is governed by a single body of Federal Circuit law. On the contrary, in fact: the court has long said that, in patent appeals, it shall default to the law of the regional circuit “unless the issue pertains to or is unique to patent law.”¹

That point is worth emphasizing because modern patent appeals rarely present only one kind of issue. A single appeal may involve venue, summary judgment, claim construction, expert admissibility, damages, collateral estoppel, and a state-law theory that overlaps with patent doctrine. The Federal Circuit’s recent opinions reflect that the court does not classify the appeal as a whole, but rather, classifies the issues within it. That is precisely why choice-of-law matters and not simply as a doctrinal preface.

The Court Applies Choice of Law Issue by Issue, Not Appeal by Appeal

One recent illustration is Focus Products Group International, LLC v. Kartri Sales Co. There, the Federal Circuit explained that venue under 28 U.S.C. § 1400(b) “is an issue unique to patent law and is governed by Federal Circuit law.”² In the same opinion, however, the court reviewed summary judgment under Second Circuit law, reviewed claim construction under Federal Circuit law, and reviewed trademark issues under Second Circuit law.³ In other words, the governing-law inquiry is not performed once at the beginning of the appeal and then forgotten. Rather, it recurs as the court moves from one issue to another.

Once an issue is classified as one governed by regional-circuit law, the Federal Circuit ordinarily adopts that circuit’s standard of review and its general procedural framework. Once an issue is classified as patent-specific, Federal Circuit law supplies the controlling rule. In mixed questions, both bodies of law may matter at once.

Damages in Particular Often Require Multiple Choice of Law Layers

This splitting-up feature of the doctrine is visible in recent expert-admissibility and damages cases.

Consider EcoFactor, Inc. v. Google LLC. There, the Federal Circuit explained that “[f]or issues not unique to patent law,” it applies the law of the regional circuit, and it proceeded to use Fifth Circuit abuse-of-discretion review for the district court’s Rule 702 ruling.

⁴ Nevertheless, the court stressed that “judicial gatekeeping is essential” and treated the district court’s “absence of reviewable reasoning” as part of the problem.⁵In other words, the regional-circuit framework governed review of the admissibility ruling, but the Federal Circuit still approached the case with a pronounced concern for the district court’s Rule 702 gatekeeping role in a patent-damages setting.

Magnolia Medical Technologies, Inc. v. Kurin, Inc. is a clean illustration of how two layers of law can combine to undo a patent verdict.⁶ The Federal Circuit began with regional-circuit law, explaining that JMOL is reviewed under Third Circuit standards, which require the evidence to be viewed in the nonmovant’s favor and permit judgment against the verdict only when the record is legally insufficient.⁷ But the court then shifted to Federal Circuit law for the dispositive issue—claim construction. There, the question was whether the claim’s separately recited “vent” and “seal” limitations could be satisfied by a single structure. Applying Federal Circuit claim-construction precedent, the panel held that “the clear implication of the claim language” was that the two limitations required distinct components, not one multipurpose part.⁸

That distinction drove the outcome. Magnolia argued that, because the parties had not separately construed “vent” and “seal” before trial, the jury was free to find infringement so long as Kurin’s porous plug performed both functions. Under a purely generic JMOL lens, that argument had some force: Magnolia had already won the verdict, and Third Circuit law required every reasonable inference in its favor. But the Federal Circuit held that the district court had not sprung a new construction on Magnolia post-verdict and had merely “only clarified what was inherent in the construction.”9 Once that patent-law rule was supplied, the factual record became fatal to infringement: the evidence was undisputed that Kurin’s porous plug served as both the vent and the seal. So even though regional-circuit law supplied the deferential JMOL framework, Federal Circuit law defined what the claim permitted the jury to find—and, on that understanding, Kurin “could not infringe as a matter of law.”¹⁰

Even Claim-Construction Is Analyzed Under the Lens of Choice of Law

Even claim construction—often viewed as quintessential patent law—may turn on state or regional circuit law for certain issues. In Trudell Medical International Inc. v. D R Burton Healthcare, LLC, the court held that the challenged expert declaration was “untethered from the district court’s claim constructions.”¹¹The admissibility question was reviewed through the lens of regional-circuit law, but the defect identified by the court was distinctly patent-specific – namely, that the expert’s noninfringement analysis did not track the court’s construction of the claims.¹²

At the same time, the Federal Circuit has explained that not every methodological criticism justifies exclusion. In Barry v. DePuy Synthes Cos., the court stated that the asserted flaws in the survey evidence “go to the weight the jury might accord to that evidence and not to its admissibility.”¹³ And Exafer Ltd. v. Microsoft Corp. reversed exclusion of a damages opinion because the methodology “is tethered to the patented invention” and therefore “satisfies the admissibility standards of Rule 702.”¹⁴ Read together, those cases show that the court is policing the line between testimony that contradicts the patent-law framework and testimony that applies that framework imperfectly but still permissibly.

Even State-Law Claims May Still Trigger Federal Circuit Law

The same issue-by-issue methodology appears when nominally state-law claims intersect with patent doctrine. In BearBox LLC v. Lancium LLC, the court stated that “Federal Circuit law governs whether federal patent law preempts a state law claim.”¹⁵ It then held that the conversion theory at issue, as pleaded, sought “patent-like protection” and was therefore preempted.¹⁶ Bearbox confirms a broader point: labels do not control. A claim styled as one arising under state law may still be governed by Federal Circuit law when the issue presented is whether the state-law theory conflicts with the federal patent scheme.

In Modern Patent Appeals, the Governing-Law Question Is Part of the Merits

The question “which law governs?” is not a boring doctrinal chore – in modern patent appeals, it is part of the substance of appellate practice. The Federal Circuit’s recent cases show that the answer may determine more than the standard of review. The court applies it issue by issue, and the choice can affect the governing standard, the standard of review, the framing of the argument, and, at times, the outcome.

Footnotes

1. 28 U.S.C. § 1295(a)(1), (4); Arctic Cat Inc. v. Bombardier Recreational Prods. Inc., No. 2017-1475, slip op. at 3-4 (Fed. Cir. Dec. 7, 2017). 

2. Focus Prods. Group Int’l, LLC v. Kartri Sales Co., Nos. 2023-1446, 2023-1450, 2023-2148, 2023-2149, slip op. at 12 (Fed. Cir. Sept. 30, 2025) (venue under 28 U.S.C. § 1400(b) is an “issue unique to patent law” and is governed by Federal Circuit law). 

3. Id. at 12-13. 

4. EcoFactor, Inc. v. Google LLC, No. 2023-1101, slip op. at 4-5 (Fed. Cir. May 21, 2025). 

5. Id. at 6, 9.

6. Magnolia Med. Techs., Inc. v. Kurin, Inc., No. 2024-2001, slip op. at 5-7 (Fed. Cir. Mar. 6, 2026).

7. Id. 

8. Id. at 9-10. 

9. Id. at 9-10. 

10. Id. at 8, 10-11. 

11. Trudell Med. Int’l Inc. v. D R Burton Healthcare, LLC, No. 2023-1777, slip op. at 5, 10-12 (Fed. Cir. Feb. 7, 2025).

12. Id.

13. Barry v. DePuy Synthes Cos., No. 2023-2226, slip op. at 9-11, 18-21 (Fed. Cir. Jan. 20, 2026).

14. Exafer Ltd. v. Microsoft Corp., No. 2024-2296, slip op. at 8 (Fed. Cir. Mar. 6, 2026) (damages methodology was “tethered to the patented invention” and therefore satisfied Rule 702).

15. BearBox LLC v. Lancium LLC, No. 2023-1922, slip op. at 11-15 (Fed. Cir. Jan. 13, 2025).

16. Id.