Secret Prior Art in 25% of Office Actions: Patently-O’s Empirical Study and the Lynk Labs Cert Denial

On April 9, 2026, Professor Dennis Crouch of the University of Missouri School of Law published an empirical study on Patently-O analyzing 9 million U.S. patents and 233 million citation records. The findings reveal that approximately 25% of USPTO office action rejections now cite “secret prior art”—references filed before but not published until after the applicant’s own filing date. Meanwhile, the U.S. Supreme Court denied certiorari on March 9, 2026 in Lynk Labs, Inc. v. Samsung Electronics Co. (125 F.4th 1120, Fed. Cir. 2025), cementing the Federal Circuit’s holding that such post-filing-date publications can be used as prior art in inter partes review (IPR) proceedings.

What Is Secret Prior Art?

Under 35 U.S.C. § 102(a)(2) (§ 102(e) under pre-AIA law), a patent application’s effective filing date serves as its prior art date against later-filed applications, even though the reference was not published until months or years later. If Applicant A files on January 1 and Applicant B files on March 1, A’s application becomes prior art against B once it eventually publishes—typically 18 months after filing. Applicant B had no way to discover A’s application when making its own filing decision. The doctrine traces to the Supreme Court’s 1926 decision in Alexander Milburn Co. v. Davis-Bournonville Co. (270 U.S. 390).

The Data: Legal vs. Practical Secrecy

Crouch’s study draws on two datasets: front-page citation records from 9 million granted U.S. patents (2002–2026) and a random sample of approximately 10,000 office actions per year from the USPTO Patent Examination Data System API (2008 through early 2026).

By the legal measure—whether the cited reference was published before the applicant filed—nearly 30% of office action rejections now rely on at least one secret reference, up from about 20% a decade ago. However, Crouch introduces a critical refinement: many “legally secret” references are continuations or divisionals whose parent applications were already published, or PCT national stage entries whose WIPO publications predated the applicant’s filing. When measured by “practical secrecy”—whether any family member had already disclosed the same content—the rate is substantially lower, though still significant.

The widening gap between legal and practical secrecy, Crouch explains, is driven by rising continuation and international filing rates. The modern patent is increasingly part of a larger family, making it more likely that a related publication already existed.

The AIPA Decline and AIA Reversal

The American Inventors Protection Act of 1999 (AIPA), effective November 29, 2000, introduced mandatory 18-month publication for most patent applications, dramatically reducing the window during which a pending application remained entirely secret. The data captures AIPA’s gradual impact: the legal secret rate fell from 25% in 2006 to 12% by 2016.

The America Invents Act (AIA), effective March 16, 2013, reversed this trend by abolishing the Hilmer doctrine (In re Hilmer, 359 F.2d 859, C.C.P.A. 1966). Under pre-AIA law, a foreign-origin application’s prior art date was limited to its U.S. filing date. The AIA extended the prior art date to the earliest worldwide filing date, including foreign priority claims. Crouch’s counterfactual analysis estimates that this expansion accounts for 3.2 additional percentage points of legally secret prior art by 2025, dropping to about 2.2 points when measured by practical secrecy.

Lynk Labs and the Supreme Court’s Silence

In Lynk Labs, Inc. v. Samsung Electronics Co. (125 F.4th 1120, Fed. Cir. 2025), the Federal Circuit held that a published patent application could serve as prior art in an IPR proceeding even though it was not publicly accessible until after the challenged patent’s filing date. The court concluded that the term “printed publication” in 35 U.S.C. § 311(b) is “temporally agnostic.”

The Supreme Court denied certiorari on March 9, 2026. In urging denial, the Solicitor General characterized the issue as having “limited practical importance.” Crouch’s data directly challenges that framing: if 25% of office action rejections rely on prior art unavailable at the time of filing, the problem is structural rather than marginal.

The same § 311(b) question remains live in VLSI Technology LLC v. Patent Quality Assurance LLC (Nos. 2023-2298, -2354, Fed. Cir.), currently pending before the Federal Circuit.

Technology Variation and Reference Characteristics

The data shows secret prior art is unevenly distributed across technology areas. Electrical engineering and telecommunications exhibit the highest rates, driven by dense filing activity in overlapping technology spaces. Mechanical engineering has the lowest. Approximately 79% of secret prior art references are published applications (pre-grant publications) with an average “secrecy gap” of 371 days. The remaining 21% are patents that were not published as applications before grant.

A counterintuitive finding: examiner-cited reference age has grown from about 4 years to over 6 years during the study period. Despite accelerating technology cycles, examiners are reaching further back in time, likely because the cumulative stock of published prior art and improved search tools give them a deeper well to draw from.

Comparative Perspective: Europe and Japan

The treatment of secret prior art varies significantly across jurisdictions. The European Patent Office (EPO) permits secret prior art to be used only for novelty, not for inventive step (obviousness). Japan’s Patent Act similarly limits its “enlarged prior application” provision (Article 29-2) to novelty-type assessments, excluding it from inventive step analysis under Article 29(2).

The U.S. stands apart: § 102(a)(2) prior art is fully available for both novelty and § 103 obviousness rejections. Since the primary use of secret prior art in U.S. examination is for obviousness, adopting the European or Japanese approach would eliminate the majority of secret-prior-art-based rejections.

Practical Implications for Applicants

Direct defenses against secret prior art are inherently limited—applicants cannot search for what does not yet exist. However, several strategic responses merit consideration. First, filing early publication requests shortens the applicant’s own secrecy window and strengthens its prior art position against competitors. Second, strategic use of continuations and divisionals increases the published footprint of a patent family, building resilience against “practically secret” references. Third, patent holders should reassess IPR vulnerability in light of Lynk Labs, particularly for patents filed in high-density technology areas.

Secret prior art represents a structural asymmetry in patent law: applicants’ rights can be denied based on information that was, by definition, undiscoverable at the time of filing. Whether and how to constrain this asymmetry is a foundational question of patent system design. The Supreme Court has, for now, chosen silence. The data suggests the question will not stay quiet for long. For related analysis of U.S. patent system developments, see our coverage of DOJ’s SEP antitrust brief in Samsung v. Netlist.