Challenging a Patent: Inter Partes Review and PTAB Practice

Inter partes review is an administrative trial in which a third party asks the Patent Trial and Appeal Board to cancel claims of an issued patent. It is not a general validity forum. The statute lets a petitioner argue only that claims are anticipated or obvious, and only using prior art that consists of patents and printed publications. Everything else a defendant might say about a patent — prior public use, on-sale activity, indefiniteness, written description, subject-matter eligibility — stays in district court.

That narrowness is the design. In exchange for a fast, specialist proceeding decided on a preponderance standard, a petitioner accepts a fixed record, a hard filing deadline, and estoppel that closes off arguments it never actually made.

What an IPR decides, and what it leaves behind

The Board decides patentability of the challenged claims. It does not decide infringement, damages, ownership, or licence disputes, and it cannot award money to anyone. A claim the Board cancels is gone against the world once appeals are exhausted; a claim the Board upholds is not blessed forever, because a different challenger with different art may try again, subject to the Board's discretion.

Two features distinguish the forum from district court. First, the burden of proving unpatentability is a preponderance of the evidence, not the clear-and-convincing standard that applies to an issued patent in court. That gap is the main reason petitioners come. Second, claims are construed under the same standard courts use — the ordinary and customary meaning to a person of skill in the art, read in light of the specification and prosecution history — after a rule change that took effect in late 2018. Before that change, the Board applied the broadest reasonable interpretation, and older commentary still describes the abandoned standard.

Post-grant review is the adjacent tool and answers different questions. It must be filed within nine months of issuance, is available only for patents examined under the first-inventor-to-file rules, and permits nearly any invalidity ground, including eligibility and written description. The two proceedings therefore fit together: post-grant review while the nine-month window is open, inter partes review afterward.

The timing gates that decide whether you can file at all

1. Day of issuance

   The patent grants. A post-grant review petition, if the patent qualifies, must be filed within nine months. An IPR petition filed in this period is premature.

2. Nine months after issuance or reissue

   The IPR door opens. If a post-grant review was instituted, the IPR window opens instead when that proceeding terminates.

3. Service of an infringement complaint

   A one-year clock starts under 35 U.S.C. § 315(b). It runs against the served party, its real parties in interest, and its privies.

4. One year after service

   The bar falls. A petition filed after this date is denied on timing alone, no matter how strong the art. Dismissal of the earlier complaint does not reliably reset the clock in every posture, so treat the date of service as final.

5. Roughly three months after the petition is accorded a filing date

   The patent owner may file a preliminary response. The Board's institution decision follows within three months of that response or of the date it was due.

6. Twelve months after institution

   The statute requires a final written decision within one year, extendable by up to six months for good cause. Appeal runs to the Federal Circuit.

One further gate is easy to trip. A petitioner that has already filed a civil action challenging the validity of the patent cannot bring an IPR. A counterclaim of invalidity filed in response to an infringement suit does not count as such an action, but a first-strike declaratory judgment complaint does. That is a direct reason to think about IPR before answering a demand letter — see how cease-and-desist correspondence shapes the forum fight.

Deadline discipline: the one-year bar runs from service of the complaint, not from the date a defendant learned of the suit, retained counsel, or completed a prior-art search. Build the petition schedule backward from service, allowing three to four months for the search, expert declaration, and claim charts.

The estoppel trade

Estoppel is the price of the lower burden. Once the Board issues a final written decision on a claim, the petitioner — along with its real parties in interest and privies — may not later assert in a district court, before the International Trade Commission, or in the Office that the claim is invalid on any ground the petitioner raised or reasonably could have raised during the review.

The phrase "reasonably could have been raised" is the operative one. It reaches patents and printed publications a skilled searcher conducting a diligent search would have found, whether or not this petitioner found them. It does not reach grounds the statute never allowed in an IPR, so prior public use, on-sale activity, and eligibility arguments survive. Petitioners frequently misjudge this line, filing a narrow petition to save cost and losing the wider paper art they wanted to keep in reserve.

Building a petition that institutes

Institution requires a reasonable likelihood that the petitioner will prevail on at least one challenged claim. If the Board institutes, it institutes on all challenged claims and all grounds in the petition — there is no partial institution — so the petition's weakest ground travels with its strongest.

• Identify every real party in interest and privy. A defective identification can sink an otherwise strong petition and can pull in someone else's one-year bar.
• Fix the claim construction positions in the petition itself. Constructions advanced later, or inconsistently with a parallel court case, invite the Board to discount them.
• Prove the prior-art references are patents or printed publications, including public accessibility for anything that is not a patent — conference papers, manuals, standards drafts, and archived web pages all need a provenance story.
• File the expert declaration with the petition, with reasoning rather than conclusions. The Board gives little weight to a declaration that repeats the petition.
• Prioritise grounds and drop the filler. Rank-ordered, non-redundant grounds read better than eight variations on one combination.
• Confirm current fees and page or word limits on the Office's site before filing; both are set by rule and adjusted periodically.

The patent owner's side has its own architecture. A preliminary response can raise procedural defects, real-party-in-interest problems, and substantive rebuttal. After institution, the owner may file one motion to amend claims, and the Board has run a pilot programme offering preliminary guidance on such motions. Settlement remains possible: parties may jointly move to terminate before a final written decision, which is one reason a licence conversation often runs in parallel with the proceeding. The commercial terms of that conversation are the subject of scope, exclusivity, and royalties in a licence.

Discretion, parallel litigation, and volatility

Even a strong petition can be denied for reasons unrelated to the art. The Board may decline to institute where a parallel district court or ITC case is likely to resolve the same validity questions first, where the same art was already considered during examination, or where serial petitions target the same claims.

This is the least stable part of PTAB practice. The Office's guidance on discretionary denial changed more than once during 2025 and remained in flux into 2026, including changes to how discretionary questions are briefed and who decides them. Any strategy that assumes a particular approach to parallel-litigation denial should be re-verified against current Director guidance on the Board's own pages immediately before filing.

Two practical consequences follow. First, speed matters independently of the statutory bar: petitions filed early relative to the court schedule fare better on discretionary grounds. Second, a district court stay motion and the IPR petition are one strategy, not two, and the stipulation a defendant offers about which grounds it will not press in court is often what tips the balance. Preservation duties attach across both tracks, so the litigation hold should cover engineering files, product history, and the search materials behind the petition.

Questions the desk gets

We were sued fourteen months ago. Is an IPR still possible?

Not by you, and not by anyone who is your real party in interest or privy. The one-year bar is jurisdictional in effect and has no good-cause extension. What remains is district court invalidity, where every ground is available but the burden is clear and convincing evidence, plus ex parte reexamination, which any person may request at any time — though the requester has no ongoing role in it and no control over the outcome.

Can a licensee challenge the patent it licenses?

Sometimes, and the licence usually decides. Many agreements contain a challenge clause that terminates the licence, raises royalties, or shifts fees if the licensee petitions. Those clauses are common enough that the question should be answered before the petition is drafted, not after. Standing to petition does not depend on being sued, so a licensee is not barred by statute — the barrier is contractual, and an exclusive license often carries the tightest version.

Does winning an IPR end the infringement case?

It ends the case as to the cancelled claims once the decision is final and appeals are resolved. If the patentee asserted other claims, other patents, or claims the Board upheld, the litigation continues on what is left. Cancellation also does not undo a judgment that has already become final, which is why the sequencing of the two tracks matters more than the outcome of either one in isolation.

What does an IPR do to damages exposure while it is pending?

Nothing directly. Damages continue to accrue during the proceeding unless a court stays the case, and the separate statutory limits on recovery keep operating in the background — including the marking rules in 35 U.S.C. § 287, which are covered in our brief on marking and the damages consequence.

Sequencing the challenge

Start with two dates: patent issuance and service of any complaint. They tell you whether the forum is open, closed, or closing. Then test whether the best invalidity theory is built on patents and printed publications. If the strongest evidence is a prior public use or an on-sale bar, the Board cannot hear it and the analysis moves to court.

If IPR is viable, run the prior-art search as though estoppel already applies, because it effectively will. Decide which grounds go in the petition knowing that what you leave out on paper art is likely lost. Coordinate the petition with any stay motion and with the district court schedule rather than filing on the last available day. And re-check current Board guidance on discretionary denial before you commit, because that is the piece most likely to have moved since the last time you looked. Related patent, licensing, and enforcement work sits on the Intellectual Property & Media desk. This brief is general information about federal procedure, not legal advice about a specific patent.