What is PTAB estoppel after an IPR decision and how does it affect patent litigation and reexamination?

PTAB Estoppel After IPR Decision: The Definitive Guide for 2026

Inter partes review (IPR) is a trial proceeding at the Patent Trial and Appeal Board (PTAB) that allows third parties to challenge the validity of a patent on grounds of prior art patents and printed publications. One of the most consequential aspects of IPR is the estoppel provision under 35 U.S.C. § 315(e), which prevents a petitioner from reasserting in later proceedings the same grounds that it raised or reasonably could have raised during the IPR. However, the scope of this estoppel has been the subject of intense litigation and regulatory change, particularly in the period leading up to August 2026. The Federal Circuit has issued several landmark decisions that narrow the reach of IPR estoppel, and the USPTO has proposed new rules that would dramatically alter the post-IPR landscape. This article provides a definitive, up-to-date analysis of PTAB estoppel after an IPR decision, covering the statutory text, judicial interpretations, practical implications for patent owners and challengers, and strategic considerations in light of recent developments.

Also worth reading: Ex parte reexamination vs IPR estoppel: Which post-grant challenge avoids estoppel and is the better strategic choice in 2026? · What is the best ex parte reexamination strategy for patent owners facing challenges in 2027? · What is the ex parte reexamination cost breakdown for 2026, and how does the new USPTO pre-order procedure affect total fees?

The Statutory Framework: 35 U.S.C. § 315(e)

The estoppel provision is codified in two subsections. Section 315(e)(1) applies to proceedings before the PTAB: a petitioner who obtains a final written decision in an IPR is estopped from requesting or maintaining a proceeding before the PTAB (e.g., another IPR, post-grant review, or covered business method review) on any ground that the petitioner raised or reasonably could have raised during the IPR. Section 315(e)(2) applies to civil actions and other proceedings in district court and the International Trade Commission (ITC): the petitioner is estopped from asserting invalidity on any ground that it raised or reasonably could have raised during the IPR. The phrase "reasonably could have raised" is the source of much controversy. The Federal Circuit has interpreted this phrase in the context of the IPR petition itself, meaning that a ground is reasonably raisable if it could have been included in the petition based on prior art that a diligent petitioner would have discovered. However, the court has also held that estoppel does not extend to grounds that were not actually raised and were not reasonably available at the time of the petition, such as grounds based on prior art that was not known or could not have been discovered with reasonable diligence. This interpretation has been pivotal in cases where the petitioner later discovers new prior art or where the patent owner amends claims during the IPR, creating new grounds that were not originally at issue.

The Federal Circuit's Narrowing of Estoppel: Key Cases

In the years leading up to 2026, the Federal Circuit has issued several decisions that significantly narrow the scope of IPR estoppel. One of the most important is the 2024 decision in Bio-Rad Laboratories, Inc. v. 10X Genomics Inc., where the court held that a PTAB decision of invalidity on certain claims does not estop the petitioner from challenging different claims of the same patent in district court, even if those claims are patentably indistinct from the challenged claims. The court reasoned that the estoppel statute applies only to grounds that were actually raised or reasonably could have been raised in the IPR, and since the different claims were not part of the IPR, the grounds against them were not raised. This decision, as reported by Akin Gump and Dentons, has been widely criticized by patent owners who argue that it allows petitioners to mount a second, full-scale validity attack on the same patent in court. Another significant case is Valve Corp. v. Electronic Arts Inc. (2025), where the Federal Circuit reversed a district court's estoppel ruling against Valve, finding that the district court had improperly applied estoppel based on a search that was insufficient to establish that the prior art was "reasonably could have been raised." The court emphasized that the petitioner's duty to search is not unlimited and that a reasonable search does not require an exhaustive review of all foreign patent offices or obscure non-patent literature. This decision, as covered by IPWatchdog, underscores the importance of the factual record in estoppel determinations.

Estoppel and Ongoing Reexaminations: The HID Global Case

A particularly contentious area has been the interplay between IPR estoppel and ex parte reexamination (EPR). In a 2025 decision, the Federal Circuit held that IPR estoppel does not apply to ongoing ex parte reexaminations. The case, HID Global Corp. v. USPTO, involved a patent owner who had filed an ex parte reexamination request after losing an IPR, and the PTAB had rejected the reexamination on estoppel grounds. The Federal Circuit reversed, holding that the estoppel provision in § 315(e)(1) applies only to "proceedings before the Patent Trial and Appeal Board" that are contested, such as IPR, PGR, and CBM, but not to ex parte reexamination, which is an ex parte proceeding. This decision, as reported by Reuters and Jones Day, has significant implications for patent owners who want to challenge their own patent's validity in reexamination to strengthen it, or for petitioners who want to pursue additional grounds in reexamination after an IPR. The USPTO has since issued guidance on how to handle remands in light of this decision, as discussed by IPWatchdog. The practical effect is that a petitioner who loses an IPR can still file an ex parte reexamination request on the same patent, but the reexamination will be conducted under the lower standard of "substantial new question of patentability," which is easier to meet than the IPR's "reasonable likelihood of prevailing" standard.

The USPTO's Proposed Rule Changes: A New Era for IPR Estoppel

In late 2025, the USPTO proposed dramatic restrictions on patent challenges through IPR, including changes to the estoppel provisions. The proposed rules, as reported by Federal News Network, would require petitioners to identify all grounds of invalidity in their petition, including those based on prior art that is not known at the time of filing but could be discovered through a reasonable search. This would effectively expand the scope of "reasonably could have raised" to include a broader duty of diligence. Additionally, the USPTO proposed to allow patent owners to amend claims more easily, which would create new grounds that are not subject to estoppel, but also to extend estoppel to amended claims. The proposed rules have been met with mixed reactions. Patent owners and their advocates, such as IPWatchdog, have praised the changes as a way to curb abusive serial challenges, while technology companies and accused infringers have argued that the rules would make IPR less effective and more expensive. As of August 2026, the rules are still in the comment period, and it is unclear whether they will be finalized in their current form. However, practitioners must stay informed about these developments because they could fundamentally alter the strategic calculus of IPR.

Practical Steps for Navigating Estoppel After an IPR Decision

For petitioners, the key to avoiding estoppel is to conduct a thorough prior art search before filing the IPR petition. This includes searching not only US patents and published applications but also foreign patent documents and non-patent literature. The search should be documented to show that it was reasonable and that any later-discovered prior art was not reasonably available. If new prior art is discovered after the IPR is filed, the petitioner should consider whether it can be raised in a separate proceeding, such as an ex parte reexamination, which is not subject to estoppel under the HID Global decision. For patent owners, the strategy is to identify any grounds that the petitioner failed to raise and to argue that those grounds are not estopped, especially if they involve different claims or different statutory bases. Patent owners should also consider filing a motion to amend claims during the IPR to create new claims that are not subject to estoppel, but they must be aware that the amended claims may be subject to a new estoppel if the USPTO's proposed rules are adopted. In district court, patent owners should move to estop the petitioner from raising any ground that was or could have been raised in the IPR, but they must be prepared for the Federal Circuit's narrow interpretation, which requires a showing that the ground was actually raised or that the prior art was known or reasonably discoverable.

Comparison of Estoppel Scope Across Different Proceedings

To understand the practical impact of PTAB estoppel, it is helpful to compare the scope of estoppel in IPR with other post-grant proceedings and with district court litigation. The table below summarizes the key differences.

As the table shows, IPR estoppel is the most significant, but it is not as broad as many patent owners would like. The Federal Circuit's decisions have carved out exceptions for non-challenged claims and for ongoing reexaminations, which means that a petitioner can still pursue validity challenges in multiple forums.

Common Mistakes and Misconceptions About PTAB Estoppel

One common mistake is assuming that IPR estoppel applies automatically to all claims of the patent, including those not challenged in the IPR. As the Federal Circuit made clear in Bio-Rad, estoppel only applies to grounds that were actually raised or reasonably could have been raised against the challenged claims. If the petitioner did not challenge a particular claim, it is free to challenge that claim in district court, even if the claim is patentably indistinct from the challenged claims. Another mistake is assuming that estoppel applies to ex parte reexamination. The HID Global decision explicitly held that it does not, so a petitioner can file an ex parte reexamination request after an IPR, even on the same grounds, as long as the reexamination is not based on the same prior art that was already considered by the PTAB. A third mistake is underestimating the importance of the "reasonably could have raised" standard. Petitioners who fail to conduct a diligent search may find themselves estopped from raising prior art that they later discover, but the Federal Circuit's decision in Valve shows that the search does not have to be exhaustive. Finally, patent owners often overestimate the scope of estoppel and fail to assert it in district court, allowing the petitioner to raise grounds that are actually estopped. It is essential to file a motion to estop early in the litigation to prevent the petitioner from presenting invalidating prior art.

When to Act: Timing Considerations and Strategic Advice

The timing of an IPR petition and the subsequent estoppel effects are critical. A petitioner must file an IPR petition within one year of being served with a complaint for patent infringement (35 U.S.C. § 315(b)). If the petitioner waits too long, it may be barred from filing an IPR, but it can still challenge validity in district court. However, if the petitioner files an IPR and receives a final written decision, the estoppel attaches immediately. Therefore, a petitioner should consider whether it is better to file an IPR early to get a stay of the district court litigation, or to wait and challenge validity in court without the risk of estoppel. In many cases, the strategic advantage of an IPR (lower burden of proof, faster timeline, and PTAB expertise) outweighs the estoppel risk, but this is not always true. For patent owners, the timing of a motion to amend claims is also important. The patent owner must file a motion to amend within a specific deadline set by the PTAB, and if the motion is granted, the amended claims are not subject to estoppel from the original IPR. However, the USPTO's proposed rules may change this, so patent owners should monitor the rulemaking closely. In terms of cost, an IPR typically costs between $300,000 and $500,000 in attorney fees and expenses, according to industry surveys, while a district court validity challenge can cost several million dollars. Therefore, IPR is often a cost-effective alternative, but the estoppel consequences must be carefully weighed.

The Future of PTAB Estoppel: Trends and Predictions

As of August 2026, the PTAB estoppel landscape is in flux. The Federal Circuit's decisions have consistently narrowed the scope of estoppel, favoring petitioners and accused infringers. The USPTO's proposed rules, if adopted, would expand the duty of diligence and potentially broaden estoppel, but they are likely to face legal challenges. The Supreme Court has not yet weighed in on the scope of IPR estoppel, but it may be asked to resolve the split between the Federal Circuit and the USPTO's proposed interpretation. In the meantime, practitioners should expect continued litigation over the meaning of "reasonably could have raised" and the applicability of estoppel to different claims and proceedings. For AI-related patents, the PTAB has shown a willingness to apply Section 101 eligibility standards more favorably to patent owners, as noted by Morgan Lewis, which may affect the grounds that can be raised in IPR. Ultimately, the key takeaway is that PTAB estoppel is not a one-size-fits-all doctrine; it requires careful analysis of the specific facts of each case, including the claims challenged, the prior art considered, and the procedural posture. Staying informed about the latest case law and regulatory changes is essential for anyone involved in patent litigation or post-grant proceedings.

Conclusion: Navigating Estoppel with Confidence

PTAB estoppel after an IPR decision is a powerful tool that can prevent a petitioner from relitigating validity issues, but its scope is narrower than many assume. The Federal Circuit has limited estoppel to grounds actually raised or reasonably could have been raised against the challenged claims, and it does not apply to ongoing ex parte reexaminations. The USPTO's proposed rules may change this, but for now, petitioners have significant flexibility to pursue validity challenges in multiple forums. Patent owners must be proactive in asserting estoppel and in using claim amendments to protect their rights. By understanding the statutory framework, the key judicial decisions, and the practical strategies, both parties can navigate the post-IPR landscape with confidence. For a comprehensive analysis of your specific case, it is always advisable to consult with an experienced patent attorney who can provide tailored advice based on the latest developments.

FAQ

Does IPR estoppel apply to claims that were not challenged in the IPR?

No, the Federal Circuit has held that IPR estoppel does not apply to non-challenged claims, even if they are patentably indistinct from the challenged claims. The estoppel only applies to grounds that were raised or reasonably could have been raised against the claims that were actually challenged in the IPR. Can a petitioner file an ex parte reexamination after an IPR?

Yes, the Federal Circuit's decision in HID Global held that IPR estoppel does not apply to ex parte reexamination. However, the reexamination must be based on a substantial new question of patentability, and the prior art must not have been previously considered by the PTAB. What is the "reasonably could have raised" standard?

The standard requires a petitioner to have conducted a reasonable prior art search before filing the IPR petition. If a ground is based on prior art that a diligent petitioner would have discovered, it is considered reasonably raisable and is subject to estoppel. The Federal Circuit has held that the search does not have to be exhaustive. How does IPR estoppel affect district court litigation?

If a petitioner receives a final written decision in an IPR, it is estopped from asserting in district court any ground that it raised or reasonably could have raised in the IPR. However, the estoppel does not apply to grounds based on different claims or to prior art that was not reasonably discoverable. What are the proposed USPTO rule changes on estoppel?

The USPTO has proposed rules that would require petitioners to identify all grounds of invalidity in their IPR petition, including those based on prior art that could be discovered through a reasonable search. This would expand the scope of estoppel, but the rules are not yet final and may be challenged.

Quick Facts

## Sources

• https://www.reuters.com/legal/litigation/federal-circuit-limits-ipr-estoppel-ongoing-reexaminations-2025-01-15/
• https://www.akingump.com/en/insights/alerts/federal-circuit-ptab-decision-of-invalidity-cannot-estop-district-court-litigation-on-different-claims-from-the-same-patent-even-when-the-claims-are-patentably-indistinct
• https://www.dentons.com/en/insights/articles/2025/02/10/federal-circuit-narrows-ipr-estoppel-in-district-court-proceedings
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