How to challenge an invalid patent claim: IPR, litigation, and other options explained?

Introduction: The Stakes of Patent Validity

A granted patent is not a presumption of validity in practice; it is a legal presumption that can be rebutted. For businesses facing infringement allegations, or for competitors seeking to clear a crowded field, the ability to challenge an invalid patent claim is a strategic necessity. The U.S. patent system provides multiple avenues for such challenges, each with distinct procedural rules, evidentiary standards, costs, and strategic consequences. The most prominent are inter partes review (IPR) before the Patent Trial and Appeal Board (PTAB), ex parte reexamination, and validity challenges in federal court litigation, either as a declaratory judgment action or as an affirmative defense in an infringement suit. Additionally, the America Invents Act (AIA) created post-grant review (PGR) for patents filed under the first-inventor-to-file regime, and covered business method (CBM) review, though the latter expired in 2020. The choice of forum is not merely tactical; it can determine the outcome. For example, the PTAB has been described by critics as a "patent death squad" due to its high invalidation rates, while district courts have historically been more deferential to patents, though recent Federal Circuit decisions have shifted the balance. Understanding the nuances of each pathway, including estoppel provisions, claim construction standards, and appeal rights, is essential for any party considering a challenge. This article provides a definitive, practical guide to challenging an invalid patent claim, drawing on recent case law, statutory changes, and procedural updates as of August 2026.

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The Direct Answer: What Are Your Options?

If you believe a patent claim is invalid, you have three primary routes: administrative challenges before the U.S. Patent and Trademark Office (USPTO), litigation in federal court, and, in limited cases, international revocation proceedings. The most common administrative route is IPR, which allows any person (except those who filed a declaratory judgment action challenging the patent more than a year prior) to petition for review of a patent's validity on grounds of prior art (anticipation or obviousness) under 35 U.S.C. §§ 102 and 103. IPR proceedings are conducted before the PTAB, which has a lower evidentiary standard (preponderance of the evidence) compared to the clear and convincing standard used in district court for issued patents. Another administrative option is ex parte reexamination, which can be filed by anyone, including the patent owner, but the requester has limited participation rights. PGR is available for patents with an effective filing date on or after March 16, 2013, and allows challenges on any invalidity ground, including subject matter eligibility and written description, but must be filed within nine months of patent grant. In litigation, a defendant can assert invalidity as a counterclaim, or a party can seek a declaratory judgment of invalidity if there is a substantial controversy. Additionally, the Patent Trial and Appeal Board's decisions can be appealed to the Federal Circuit, and district court findings of invalidity are also appealable. Each option has different timelines, costs, and estoppel effects. For example, IPR must be filed within one year of being served with a complaint for infringement, and if the PTAB issues a final written decision, the petitioner is estopped from raising the same grounds in district court or before the USPTO. This article will dissect each option, providing practical steps and strategic considerations.

How to Challenge a Patent Claim via Inter Partes Review (IPR)

IPR is the most frequently used administrative challenge, with over 1,200 petitions filed annually in recent years. To initiate an IPR, you must file a petition with the PTAB that identifies each claim challenged, the specific grounds, and the evidence supporting the challenge. The petition must be filed within one year of being served with an infringement complaint, unless you have the patent owner's consent or a legitimate reason for delay. The PTAB then has three months to decide whether to institute review, applying a "reasonable likelihood that the petitioner would prevail" standard. If instituted, the PTAB issues a final written decision within 12 months (extendable by six months for good cause). The standard of proof is preponderance of the evidence, meaning the petitioner must show it is more likely than not that the claim is invalid. This is a lower bar than the clear and convincing evidence required in district court. However, the PTAB's claim construction standard is different: it uses the "broadest reasonable interpretation" (BRI) for patents that have not expired, which can make it easier to find invalidity. After the final decision, the petitioner is estopped from asserting in district court or before the USPTO any ground that it raised or reasonably could have raised during the IPR. This estoppel is a double-edged sword: it provides finality but can limit later litigation strategies. Notably, the Federal Circuit has recently clarified that estoppel applies to grounds that were actually raised, not necessarily all grounds that could have been raised, but the scope remains contested. For example, in a 2025 decision, the Federal Circuit held that a petitioner who raised an obviousness ground in IPR could not later assert a different combination of prior art in district court if it was the same ground. Therefore, careful drafting of the IPR petition is critical to avoid inadvertently waiving arguments. Practical steps include conducting a thorough prior art search, preparing expert declarations, and considering whether to file a preliminary response. The cost of an IPR typically ranges from $300,000 to $500,000, including attorney fees and expert costs, but can be higher for complex technologies.

Litigation as a Challenge: Declaratory Judgment and Infringement Defenses

In federal court, challenging a patent's validity can occur in two contexts: as a declaratory judgment action (where the challenger is the plaintiff) or as an affirmative defense in an infringement suit (where the challenger is the defendant). To bring a declaratory judgment action, you must demonstrate an actual controversy, such as receiving a cease-and-desist letter or being threatened with litigation. The advantage of being the plaintiff is that you control the forum and timing, but you must be prepared to prove invalidity by clear and convincing evidence, which is a higher standard than in IPR. In contrast, an accused infringer can raise invalidity as a counterclaim, which is a compulsory counterclaim if it arises from the same transaction. The district court will apply the presumption of validity, and the challenger must present evidence that is "clear and convincing" to overcome it. This standard is difficult to meet, but recent Supreme Court decisions, such as Microsoft v. i4i (2011), have upheld this standard, making litigation a less attractive option for weak invalidity arguments. However, litigation offers broader discovery, including the ability to depose the inventors and examine the prosecution history, which can uncover evidence of inequitable conduct or prior art not considered by the examiner. Additionally, a jury trial is available, and juries may be more sympathetic to challengers in certain cases. The cost of patent litigation is substantial, with median costs exceeding $1 million through trial, and can reach $4 million or more for complex cases. One strategic consideration is the timing of the challenge: if you file an IPR, the district court may stay the litigation pending the PTAB's decision, which can save costs. However, if you lose the IPR, you may be estopped from raising the same grounds in litigation, so you must choose your grounds carefully. A recent trend is the use of "skeleton" IPR petitions to preserve arguments, but this is risky due to estoppel. The Federal Circuit has also raised the bar for patent owners seeking to avoid estoppel, as noted in a 2026 decision, making it harder to relitigate issues.

Ex Parte Reexamination and Post-Grant Review (PGR)

Ex parte reexamination is a less common but lower-cost alternative. Any person, including the patent owner, can request reexamination of a patent based on prior art patents or printed publications. The request must raise a "substantial new question of patentability" (SNQ), and the examiner will determine whether to grant the request. If granted, the patent owner can file a response, but the requester has no further participation rights. The proceeding is conducted ex parte, meaning the requester is not involved after the initial request. The standard of proof is preponderance of the evidence, and the examiner can invalidate claims or confirm their validity. The cost is significantly lower than IPR, typically $50,000 to $100,000, but the lack of participation can be a disadvantage. PGR, on the other hand, is a more powerful tool but is limited to patents with an effective filing date after March 16, 2013. PGR must be filed within nine months of the patent grant, and it allows challenges on any ground of invalidity, including subject matter eligibility (35 U.S.C. § 101), written description, enablement, and indefiniteness. The standard for institution is "more likely than not" that at least one claim is unpatentable, which is a higher threshold than IPR's "reasonable likelihood." PGR also has estoppel provisions similar to IPR, but they apply to all grounds that were raised or reasonably could have been raised. PGR is often used for software and business method patents, where eligibility challenges are common. However, the nine-month window is short, so you must act quickly after a patent issues. As of 2026, the PTAB has seen a decline in PGR filings due to the restrictive institution standard, but it remains a viable option for early-stage challenges. A comparison of these administrative options is provided in the table below.

The choice between IPR, PGR, litigation, or reexamination depends on several factors: the patent's filing date, the grounds of invalidity, the stage of litigation, and the budget. For patents with an effective filing date before March 16, 2013, IPR is the only administrative option for prior art challenges, as PGR is unavailable. If the patent is less than nine months old, PGR offers a broader range of grounds, but the higher institution standard means you need a strong case. In litigation, you can challenge validity on any ground, but the clear and convincing standard is a significant hurdle. A common strategy is to file an IPR while simultaneously defending against infringement in district court, and then request a stay of the litigation. Courts are more likely to grant a stay if the IPR is instituted and if the litigation is in its early stages. However, the estoppel risk must be weighed: if you lose the IPR, you cannot raise the same grounds in litigation, but you can still raise different grounds. For example, if you challenge a claim as obvious over prior art A and B in IPR, you cannot later argue obviousness over A and C in district court if C was known and could have been raised. This was the holding in a 2025 Federal Circuit case, which emphasized that "reasonably could have raised" includes prior art that a skilled searcher would have found. Therefore, it is essential to conduct a comprehensive prior art search before filing an IPR to identify all potential grounds. Another consideration is the PTAB's discretion to deny institution under 35 U.S.C. § 314(a), as articulated in the NHK-Fintiv factors, which consider whether a parallel district court proceeding is scheduled to go to trial before the PTAB's final decision. In 2026, the PTAB has been more willing to deny IPR petitions when the district court trial is imminent, so timing is critical. Additionally, the proposed rule changes to PTAB procedures, as discussed by Mayer Brown, could limit IPR challenges by requiring more detailed claim charts and reducing the number of claims that can be challenged. These changes are not yet final, but they signal a shift toward making IPR more difficult.

Common Mistakes to Avoid When Challenging a Patent

One of the most common mistakes is failing to conduct a thorough prior art search before filing an IPR. Many petitioners rely on a few references they found in a quick search, only to discover later that a better reference existed, but they are now estopped from using it. Another mistake is misidentifying the claims or failing to provide a detailed claim chart that maps each limitation to the prior art. The PTAB requires a clear explanation of how the prior art teaches each element, and vague arguments are often rejected. Additionally, petitioners often overlook the one-year bar for IPR after being served with an infringement complaint. If you wait too long, you lose the ability to file an IPR, and you are left with litigation only. Another error is ignoring the patent owner's preliminary response, which can include expert testimony and arguments that undermine the petition. The PTAB may deny institution based on the response, so you must anticipate and rebut these arguments in your petition. In litigation, a common mistake is failing to raise invalidity as a counterclaim, which can result in waiver. Under Federal Rule of Civil Procedure 13(a), invalidity is a compulsory counterclaim if it arises from the same transaction, so you must assert it or lose the right. Another mistake is relying solely on obviousness arguments without considering anticipation, or vice versa. The best strategy is to present multiple grounds, but be mindful of estoppel. Finally, many challengers underestimate the cost and time required. IPR proceedings can take over a year, and litigation can take years, so you need a long-term budget. A critical mistake is not considering the patent owner's ability to amend claims during IPR. The patent owner can file a motion to amend, and if the amendment is granted, the challenged claims may be replaced with narrower claims that avoid the prior art. You must be prepared to challenge the amended claims as well.

When to Act: Timing and Deadlines

Timing is everything in patent challenges. The most critical deadline is the one-year IPR bar: if you are served with an infringement complaint, you have one year to file an IPR petition. This deadline is strict, and extensions are rarely granted. For PGR, the deadline is nine months from the patent grant, which can be earlier than the one-year bar if the patent is recently issued. Therefore, you must monitor the patent's issuance date and act quickly. In litigation, the timing is governed by the court's scheduling order, but you should raise invalidity as early as possible to avoid waiver. A declaratory judgment action can be filed as soon as there is an actual controversy, which may be before the patent owner files suit. However, if you file a declaratory judgment action, you cannot later file an IPR on the same patent, as the AIA prohibits IPR by a party that has already filed a declaratory judgment action challenging the patent. This is a trap for the unwary. Another timing consideration is the PTAB's discretion to deny institution based on the Fintiv factors. If a district court trial is scheduled to begin before the PTAB's final decision, the PTAB may deny the IPR, so you may need to file the IPR early in the litigation. Conversely, if you file the IPR too early, before the patent owner has filed suit, you may not have the benefit of the patent owner's infringement contentions, which can inform your claim construction. A balanced approach is to file the IPR within a few months of being sued, but after you have had time to conduct a thorough prior art search. In 2026, the PTAB has been more aggressive in denying IPR petitions that are filed after the district court has issued a claim construction order, so it is wise to file before that point. Finally, consider the patent's expiration date. If the patent is near expiration, the cost of a challenge may not be justified, as the patent will soon be unenforceable. However, if the patent owner is seeking damages for past infringement, a validity challenge may still be worthwhile.

Cost and Pricing: What to Expect

The cost of challenging a patent varies widely depending on the forum and complexity. IPR proceedings typically cost between $300,000 and $500,000, including attorney fees, expert witness fees, and the USPTO filing fee (which is $41,500 for IPR as of 2026). This cost can be higher for complex technologies, such as pharmaceuticals or software, where expert testimony is essential. PGR is slightly more expensive, ranging from $400,000 to $600,000, due to the broader scope of challenges. Ex parte reexamination is the cheapest administrative option, costing $50,000 to $100,000, but it offers limited participation. Litigation is the most expensive, with median costs of $1.5 million through trial, and can exceed $4 million for high-stakes cases. These costs include discovery, depositions, motions, and trial preparation. For small businesses, the cost may be prohibitive, but there are alternatives, such as joining a collective challenge like the Electronic Frontier Foundation's Patent Busting Project, which targets patents that are illegitimate. However, such projects are limited in scope. Another cost-saving strategy is to file a petition for IPR with a limited number of claims, as the PTAB fees are per claim. For example, the fee for the first 20 claims is included, but each additional claim costs $200. You can also request a fee reduction if you qualify as a micro-entity. It is important to budget for the possibility of an appeal to the Federal Circuit, which can add $200,000 to $500,000 in additional costs. In litigation, you may be able to recover attorney fees if you prevail and the case is exceptional under 35 U.S.C. § 285, but this is rare. Overall, the cost should be weighed against the potential damages or market exclusivity at stake. If the patent is blocking your product, the cost of a challenge may be justified, but if the patent is weak and you can design around it, a challenge may not be worth the expense.

Recent Developments and Future Outlook

As of August 2026, several developments are shaping the landscape of patent challenges. The Federal Circuit has continued to refine estoppel rules, with a 2026 decision holding that a petitioner who raised an obviousness ground in IPR cannot later assert a different combination of prior art in district court if the combination was known and could have been raised. This decision has been criticized for being too broad, but it underscores the importance of comprehensive prior art searches. Additionally, the PTAB has been implementing new rules proposed in 2025, which may require petitioners to provide more detailed claim charts and limit the number of claims that can be challenged. These rules are intended to reduce the burden on the PTAB, but they may make IPR more difficult and expensive. The Senate has held hearings on patent reform, with a focus on software and AI patents, and there is bipartisan support for raising the bar on patent eligibility, which could affect challenges to such patents. In the meantime, the PTAB remains a popular forum, with a 2026 study showing that 65% of IPR petitions result in at least one claim being invalidated. However, the PTAB's discretion to deny institution under Fintiv has led to a decline in IPR filings in cases where parallel litigation is ongoing. The Supreme Court has not taken up any major patent validity cases recently, but the Federal Circuit's decisions are providing clarity. For challengers, the key takeaway is to stay informed about procedural changes and to act early. The future may see a shift toward more centralized patent validity challenges, such as a unified patent court, but for now, the U.S. system remains fragmented. In any case, challenging an invalid patent claim requires a strategic, well-funded approach, and the choice of forum can make or break your case.

Conclusion: Practical Steps to Take

To challenge an invalid patent claim, start by conducting a thorough prior art search and identifying all potential grounds of invalidity. If the patent is less than nine months old and has an effective filing date after March 16, 2013, consider filing a PGR. If the patent is older, IPR is the primary administrative option, but be mindful of the one-year bar. If you are already in litigation, you can raise invalidity as a counterclaim, but you must also consider whether to file an IPR and request a stay. Consult with a patent attorney who specializes in PTAB proceedings, as the procedural rules are complex. Prepare a detailed petition with expert declarations and claim charts. Budget for the costs, and consider the estoppel implications. Finally, monitor the patent's expiration date and the PTAB's institution decisions. By following these steps, you can effectively challenge an invalid patent claim and protect your business interests.

## FAQ What is the difference between IPR and PGR?

IPR is available for any patent and can only challenge claims on prior art grounds (anticipation or obviousness) under 35 U.S.C. §§ 102 and 103. PGR is only available for patents with an effective filing date after March 16, 2013, and must be filed within nine months of grant. PGR allows challenges on any invalidity ground, including subject matter eligibility and written description. The institution standard for PGR is higher ("more likely than not") compared to IPR's "reasonable likelihood." Can I challenge a patent after being sued for infringement?

Yes, you can file an IPR within one year of being served with an infringement complaint. You can also raise invalidity as a counterclaim in the litigation. However, if you file a declaratory judgment action before being sued, you are barred from filing an IPR. The choice between IPR and litigation depends on the grounds and the desired speed. What is the estoppel effect of an IPR?

If the PTAB issues a final written decision in an IPR, the petitioner is estopped from asserting in district court or before the USPTO any ground that it raised or reasonably could have raised during the IPR. This means you cannot relitigate the same prior art combinations. The scope of estoppel has been clarified by recent Federal Circuit decisions, but it is broad. How much does it cost to challenge a patent?

IPR costs typically range from $300,000 to $500,000, including attorney fees and expert costs. PGR is slightly more expensive, at $400,000 to $600,000. Ex parte reexamination is cheaper, at $50,000 to $100,000, but offers limited participation. Litigation is the most expensive, with median costs exceeding $1.5 million through trial. What are the common mistakes to avoid when challenging a patent?

Common mistakes include failing to conduct a thorough prior art search, missing the one-year IPR deadline, providing vague claim charts, and ignoring the patent owner's preliminary response. In litigation, failing to raise invalidity as a counterclaim can result in waiver. Also, be aware of the PTAB's discretion to deny institution based on parallel litigation.

Quick Facts

• Category: Patent Validity Challenge
• Timeline: IPR must be filed within 1 year of infringement complaint; PGR within 9 months of grant; litigation can be filed anytime with actual controversy.
• Cost: IPR: $300k-$500k; PGR: $400k-$600k; Ex Parte Reexamination: $50k-$100k; Litigation: $1.5M+.
• Best for: IPR is best for prior art challenges; PGR for broad grounds on newer patents; litigation for comprehensive discovery and jury trial.
• Success Rate: PTAB invalidates at least one claim in about 65% of IPR petitions (2026 study).
• Estoppel: IPR and PGR have estoppel; ex parte reexamination does not.

Sources

• https://www.akingump.com/en/insights/aliases/invalidity-defense-raised-prior-to-ptab-challenge-cannot-skirt-estoppel
• https://ipwatchdog.com/2026/01/15/despite-recent-changes-ptab-remains-patent-death-squad/
• https://www.mayerbrown.com/en/insights/publications/2025/12/proposed-revision-to-ptab-rules
• https://www.law360.com/articles/2026/02/10/fed-circ-backs-ptab-invalidation-of-videolabs-patent-claims
• https://www.biospace.com/geneoscopy-wins-second-ptab-decision-invalidating-all-challenged-claims-of-exact-sciences-746-patent
• https://www.jdsupra.com/legalnews/appellate-court-raises-the-bar-for-patent-owners-pursuing-estoppel-2026/
• https://www.hunton.com/insights/a-recent-change-in-patent-office-procedures-makes-challenging-patents-more-difficult
• https://www.eff.org/patent-busting-project

Follow-up Keyword

PTAB estoppel after IPR decision