Reexamination vs. Other Post-Grant Proceedings: A Comparative Analysis of Patent Challenge Options

You’ve uncovered prior art that could torpedo a rival’s patent. Now comes the decision that could define your litigation strategy: Which post-grant path gives you the strongest footing without boxing you into estoppel or tipping your hand too early?

The USPTO provides three primary options—inter partes review (IPR), post-grant review (PGR), and ex parte reexamination. Each comes with its own rules, limitations, and tactical implications that can affect not just the current challenge, but future litigation strategy as well.

The key differences come down to timing, scope, and control. IPR challenges are limited to novelty and obviousness and can only be filed after nine months from patent grant. PGR allows broader arguments, including subject matter eligibility and written description, but only within that first nine-month window. Ex parte reexamination is available at any time, but once filed, the challenger plays no further role in the proceeding.

That’s where the real challenge begins: translating procedural differences into patent defense strategies. Let’s break down how each path shapes your odds.

Key Takeaways

• Each option offers distinct levels of procedural involvement and influence over the outcome.
• Filing windows and the available grounds for invalidity are critical early decision points.
• Cost efficiency, timing, and estoppel exposure weigh heavily in the optimal path forward.

Fundamental Distinctions Between Post-Grant Proceedings

The America Invents Act introduced a trio of proceedings to challenge patent validity at the USPTO. These differ in when they can be initiated, what arguments they permit, and how much they allow challengers to engage.

Ex Parte Reexamination Overview

Ex parte reexamination gives anyone the chance to question patent validity using only prior art in the form of patents or printed publications. The process is handled privately by the USPTO, and once the request is submitted, the requester plays no further active role.

The examiner reevaluates the patent based solely on the cited prior art. Patent owners may propose claim amendments throughout the process.

The threshold for review is whether a substantial new question of patentability exists. Importantly, this route does not create estoppel, leaving the door open for future litigation patent defense strategies.

Inter Partes Review Fundamentals

Inter partes review becomes available nine months after a patent issue. It focuses solely on grounds of novelty and obviousness, relying on prior art documents.

The Patent Trial and Appeal Board (PTAB) conducts these proceedings like a mini-trial, complete with discovery, expert declarations, and oral hearings. Both sides participate from start to finish.

What sets IPR apart:

• Decisions are typically reached within 12 months.
• Uses the preponderance of the evidence standard.
• Triggers estoppel for all grounds raised or reasonably available in future venues.

Post-Grant Review Essentials

Post-grant review must be filed in the first nine months after the patent grant and permits a broader range of invalidity grounds, such as lack of enablement, indefiniteness, and patent-ineligible subject matter.

To proceed, the challenger must show that the claims are more likely than not invalid on at least one ground.

Like IPR, PGR includes:

• Rigorous standing requirements
• Limited opportunities to amend claims
• Strong estoppel consequences post-decision

Strategic Selection Criteria For Patent Challengers

Choosing the best challenge route hinges on a mix of timing, evidentiary leeway, and tolerance for risk. Decisions made in the early stages ripple through to final patent litigation outcomes. A mistake here can weaken your strategy in future litigation or trigger estoppel limitations. 

Timing Considerations

Timing can be a gatekeeper. PGR is off the table after nine months, while IPR opens only after that window closes. If your prior art surfaces within those first nine months, PGR gives you a broader attack surface. Miss that chance and you automatically narrow your options. 

Ex parte reexamination is not tied to any fixed deadline, which makes it a flexible option—especially when new prior art comes to light after litigation is underway. 

Also factor in how long each proceeding typically takes. IPR usually wraps up within a year, while ex parte reexamination may stretch to 2–3 years, potentially overlapping with court timelines.

Evidentiary And Procedural Advantages

IPR and PGR allow expert reports and in-depth legal arguments. This creates room to develop a full evidentiary record to support complex invalidity theories. The ability to build a rich record can provide a significant advantage in parallel litigation.

By contrast, ex parte reexamination confines arguments to patents and printed publications. This narrower evidence scope can work well for clean, prior-art-based challenges.

The discovery landscape also varies:

• IPR/PGR: Some access to documents and cross-examination
• Ex parte: No discovery
• PGR uniquely permits challenges under a broader statutory range

Estoppel Implications

A key difference lies in estoppel. IPR and PGR both prevent challengers from raising, in court or later proceedings, any argument that was or could have been made during the process. That’s a major strategic constraint.

A weak challenge could box you in and eliminate stronger arguments later when the stakes are even higher. Don’t go into IPR or PGR unless you are confident in your invalidity theories and patent defense strategies.

Ex parte reexamination sidesteps this problem. It does not bind the challenger in later proceedings, making it a useful tool for probing weaknesses without closing off litigation options. It would work well as a testing ground. 

Importantly, estoppel doesn’t just stick to the named challenger. It may extend to affiliated parties, creating broader risks that must be managed. When working as part of a corporate group or coordinating with codefendants, everyone involved should understand these estoppel consequences. 

Real-World Estoppel Application

Courts can interpret estoppel broadly, even excluding arguments based on nuanced differences. Consider the Wasica Finance GmbH v. Schrader International, Inc. 2020 case. The court held that the defendant was estopped from asserting invalidity arguments based on prior art that was not presented during the IPR but could have been found through a diligent search. The court emphasized that estoppel applies to any ground that the petitioner “reasonably could have raised” during the IPR, reinforcing a broad interpretation of estoppel provisions.

That raises the stakes for anyone considering IPR or PGR.

To manage this, practitioners often spread their arguments across proceedings. One common tactic is to use ex parte reexamination to vet weaker arguments early and save the strongest ones for IPR.

Another involves using different challengers, such as suppliers or partners, to maintain flexibility across multiple fronts. These approaches have helped challengers successfully navigate estoppel while pressing their case.

Tactical Considerations For Patent Owners And Challengers

Thoughtfully selecting and sequencing post-grant challenges can influence litigation posture, settlement timing, and the strength of claims going forward.

Litigation Interface Strategies

Where IPR is instituted, district courts grant litigation stays in a majority of cases. This pause can shift momentum and reduce legal costs while giving the challenger more time to build pressure.

However, arguments made in IPR or PGR can backfire later. For instance, failing to raise a key point might result in losing the ability to argue it in court.

Smart challengers weigh when and how to press their strongest invalidity contentions. Often, they’ll front-load IPR with the best arguments and hold lesser ones in reserve for other venues.

Success Rate Comparisons

Statistics show IPR is the most potent challenge option, with full invalidation in almost 80% of cases upon reaching a final written decision. Ex parte reexamination sees fewer complete cancellations.

PGR opens the most comprehensive challenge grounds, but it’s harder to get past the institution threshold due to its broad scope.

Patent owners stand a better chance in ex parte proceedings, where there’s no opposing counsel and they can engage directly with examiners.

Patent Owner Response Strategies

A proactive approach benefits patent holders. Spotting weaknesses early gives them time to prepare detailed preliminary responses that point out technical errors or procedural missteps in petitions.

Motions to amend—when filed early—also offer a chance to preserve valuable claims. This can sometimes mean the difference between saving a patent and losing it entirely.

Experts can strengthen the patent owner’s case, especially when presenting real-world evidence of non-obviousness, such as commercial success or industry praise.

Where appropriate, patent owners might file a reissue or reexamination request to fix vulnerable claims ahead of anticipated challenges, tightening their position before litigation heats up.

Choose the Right Patent Challenge to Strengthen Your Position

Each post-grant option has its own benefits and trade-offs. IPR offers a fast timeline, full participation, and high success rate. But it can trigger strong estoppel and limit you to prior art challenges. PGR allows the broadest challenge grounds, but is only available in the first nine months and is harder to get instituted. Ex parte reexamination is low-risk with no estoppel and flexible timing, but the requester has no further role after filing, and the process is slower.

Making the wrong choice early on can weaken your legal position and limit future litigation options. Need help choosing the right path to protect the validity of a patent or develop smart patent defense strategies? Talk to us for experienced and strategic guidance.