Overview: ways to challenge patent validity
After a patent issues, its validity can be challenged in several ways: (1) Ex parte reexamination — anyone files a request with the USPTO based on prior art; limited third-party participation after filing; no estoppel for the requestor. (2) Inter partes review (IPR) — contested adversarial proceeding before PTAB for patents on utility and design patents; must be filed within one year of being served an infringement complaint; full estoppel consequences. (3) Post-grant review (PGR) — like IPR but can raise any invalidity ground (not just prior art); must be filed within 9 months of grant; only available for AIA patents. (4) Covered Business Method review (CBM) — now expired for most purposes, covered financial product patents. (5) Federal court — district court litigation where invalidity is raised as a defense; requires clear and convincing evidence standard (higher than PTAB).
Ex parte reexamination
Ex parte reexamination (35 U.S.C. §§ 302–307) allows any person to request that the USPTO reexamine an issued patent based on prior art patents and printed publications. The requesting party must establish a 'substantial new question of patentability' — that the cited prior art raises a new question not previously considered by the examiner. The USPTO grants reexamination in roughly 90% of cases because the threshold is relatively low. Once granted, the reexamination is conducted primarily between the examiner and the patent owner; the third-party requester has limited further participation (the ability to file one reply to the patent owner's response). Ex parte reexamination is relatively inexpensive and does not carry IPR's estoppel consequences, making it a useful tool for anonymous invalidation attempts or when a challenger wants to preserve full district court options.
Inter partes review (IPR)
IPR replaced inter partes reexamination under the America Invents Act (AIA, 2012). An IPR petition must identify all real parties in interest, specify the challenged claims, provide claim charts mapping prior art to each claim, and show a 'reasonable likelihood' of prevailing on at least one challenged claim. The PTAB institutes (or denies) the IPR within 3 months of the petition. If instituted, the proceeding is adversarial: both sides can take limited discovery, file briefs, and appear for oral argument. A final written decision must issue within 12 months of institution. Fees: the USPTO charges $19,500 for a petition covering up to 20 claims ($750/additional claim). Attorney fees typically add $60,000–$150,000+. The result: approximately 50–65% of all claims challenged in instituted IPRs are cancelled or amended.
Post-grant review (PGR)
Post-grant review (35 U.S.C. §§ 321–329) is broader than IPR in the grounds it can cover — PGR allows challenges based on any invalidity ground under 35 U.S.C. §§ 101, 102, 103, and 112 (not just prior art, as with IPR). PGR petitions must be filed within 9 months of the patent's grant date, making it a relatively narrow window. PGR is available only for patents that have a first effective filing date on or after March 16, 2013 (AIA patents). PGR estoppel is similar to IPR estoppel but covers grounds that 'reasonably could have been raised.' Because PGR allows § 101 (patent eligibility) challenges in addition to prior art, it is particularly useful for challenging software and business method patents that may be vulnerable to Alice doctrine rejections.
The one-year bar for IPR
Under 35 U.S.C. § 315(b), an IPR petition may not be filed more than one year after the petitioner (or a real party in interest in privity with the petitioner) is served with a complaint alleging infringement of the patent. This is the most important deadline in IPR strategy. A defendant in a patent lawsuit must decide quickly whether to file an IPR — waiting more than a year forecloses the option. The one-year bar applies even if the complaint was later dismissed without prejudice, with some exceptions. This bar does not apply to ex parte reexamination, which can be filed at any time. There is also a parallel bar: if a petitioner files an IPR after being served a complaint, and then the district court case is stayed pending the IPR, the estoppel at the end of the IPR will prevent re-litigating the same invalidity grounds in court.
IPR estoppel consequences
IPR estoppel (35 U.S.C. § 315(e)) is the most significant strategic consequence of filing an IPR. If a petitioner participates in an IPR, and the PTAB issues a final written decision, the petitioner (and parties in privity) are estopped from asserting in any USPTO proceeding, district court, or ITC proceeding that any claim addressed in the IPR final written decision is invalid on any ground the petitioner raised or reasonably could have raised during the IPR. This means: if you file an IPR on prior art grounds A and B, and the claims survive, you cannot later argue in district court that the claims are invalid based on prior art A, B, or any art you reasonably could have found. This 'reasonably could have raised' language is broadly construed. Estoppel does not apply if the IPR petition is denied institution.
PTAB vs. district court: comparing the forums
IPR/PTAB proceedings differ from federal district court invalidity challenges in several important ways. Burden of proof: PTAB uses preponderance of the evidence (more likely than not invalid); district courts require clear and convincing evidence for invalidity (a significantly higher standard). Claim construction: PTAB uses the Phillips standard (plain meaning to a POSITA in view of the specification), the same as district courts. Timeline: PTAB targets a final written decision within 12 months of institution; district court litigation from complaint to trial typically takes 2–4 years. Cost: IPR proceedings are typically $100,000–$350,000 in legal fees; district court litigation through trial costs $3M–$10M+. Scope: PTAB (for IPR) is limited to prior art and printed publications; district court can hear any invalidity argument including § 101, § 112, and prior public use.
Strategic considerations for patent challengers
Companies facing patent infringement claims must choose between IPR, ex parte reexamination, and district court invalidity defense — or some combination. Key considerations: (1) Speed — IPR reaches a decision faster than district court. (2) Estoppel risk — IPR forecloses re-litigating the same grounds; a loss in IPR makes district court defense harder. (3) Success rates — PTAB historically invalidates a significant portion of challenged claims; district courts have a lower patent invalidation rate. (4) Cost — IPR is cheaper than district court. (5) Anonymity — ex parte reexamination can be filed anonymously (through a third-party requester), preserving strategic flexibility. A common strategy: file IPR within the one-year window as a defensive measure, while simultaneously raising invalidity as a defense in district court, then move to stay the district court case pending the IPR outcome.
Frequently Asked Questions
What is patent reexamination?
Patent reexamination is an administrative proceeding at the USPTO that allows an issued patent to be reexamined for validity based on prior art patents and printed publications. There are two forms: ex parte reexamination (filed by anyone, including the patent owner or a third party, but conducted primarily between the USPTO and the patent owner with limited third-party participation) and inter partes review (IPR), which is a contested proceeding before the Patent Trial and Appeal Board (PTAB) with full adversarial participation. Post-grant review (PGR) is a third option available only within 9 months of patent grant. Reexamination and IPR are typically faster and less expensive than federal court litigation, but they carry different strategic implications.
What is the difference between ex parte reexamination and inter partes review (IPR)?
Ex parte reexamination (35 U.S.C. §§ 302–307) can be requested by anyone at any time based on prior art patents and printed publications; after filing, the third party has limited further participation and the proceeding is primarily conducted between the examiner and patent owner. A third party requesting ex parte reexamination does not face estoppel in subsequent litigation if the request is granted. Inter partes review (IPR, 35 U.S.C. §§ 311–319) is a fully contested adversarial proceeding before a PTAB panel; a petitioner who participates in an IPR is estopped from later asserting in district court that any claim addressed in the IPR is invalid on any ground the petitioner raised or reasonably could have raised. IPR must be filed within one year of being served with a complaint for patent infringement, or cannot be filed at all.
What is the one-year bar for filing IPR?
Under 35 U.S.C. § 315(b), a petition for inter partes review may not be filed more than one year after the petitioner (or a party in privy with the petitioner) is served with a complaint alleging infringement of the patent. This is called the one-year bar. If a defendant in a patent lawsuit wants to file an IPR as a litigation strategy, it must do so within one year of being served. There is no one-year bar for ex parte reexamination, which can be filed at any time during the patent's term.
What is the threshold to institute an IPR?
The PTAB will institute (allow to proceed) an IPR only if the petition establishes a 'reasonable likelihood that the petitioner would prevail with respect to at least one claim challenged in the petition.' This is a preponderance of the evidence standard for the preliminary inquiry. Once instituted, the IPR proceeds with briefing, optional oral argument, and a final written decision from a three-judge PTAB panel typically within 12 months of institution. Historically, roughly 60–70% of IPR petitions have been instituted, and of those that are instituted, the majority of challenged claims are cancelled or amended — IPR has been a highly effective tool for patent challengers.
What is IPR estoppel and why does it matter strategically?
IPR estoppel (35 U.S.C. § 315(e)) prevents a petitioner who participates in an IPR from later challenging the same patent claims in district court on any ground the petitioner raised or reasonably could have raised during the IPR. This is a significant strategic consequence: if a company challenges a patent in IPR and loses (or the IPR results in the claims surviving), it cannot re-litigate those validity grounds in district court. This means challengers must think carefully about IPR strategy — filing an IPR forecloses alternative invalidity arguments. Some defendants choose to litigate validity in district court rather than risk estoppel. Others file IPR to get the faster PTAB timeline and the lower burden of proof (preponderance vs. clear and convincing evidence in district court).