What is the difference between inter partes review and post-grant review?

Inter partes review (IPR) and post-grant review (PGR) are the two principal trial proceedings conducted at the Patent Trial and Appeal Board (PTAB) to challenge the validity of claims in an already-issued patent. The Leahy-Smith America Invents Act (AIA) added post-grant review as a new proceeding and renamed the existing inter partes reexamination procedure as inter partes review. The inter partes review procedure took effect on September 16, 2012, as did post-grant review. They are not interchangeable: each occupies a distinct phase of the post-grant lifecycle, covers different invalidity grounds, and carries different strategic consequences for a petitioner who reaches a final written decision.

The filing window: a sequential structure

The most fundamental structural difference between IPR and PGR is timing.

PGR occupies the early post-grant phase. A PGR petition must be filed within a strict statutory window measured from the date the patent grants or a reissue patent issues; once that window closes, PGR is permanently gone for that patent.

IPR becomes available only after the PGR window closes. For patents subject to the first-inventor-to-file provisions of the AIA, an IPR petition cannot be filed during the PGR window; the petitioner must wait until the PGR window has expired or until any PGR proceeding on that patent has been terminated.

The practical result is a two-phase structure: challengers who need the broader PGR grounds must act quickly after grant. Once the PGR window passes, those grounds are permanently unavailable at the PTAB.

Grounds for challenge: the most important substantive difference

The scope of permissible invalidity grounds differs significantly between the two proceedings, and this is usually the more consequential distinction.

IPR is a prior-art-only mechanism. Challenges are limited to grounds that could be raised under 35 U.S.C. sections 102 or 103, and only on the basis of prior art consisting of patents or printed publications. Novelty and obviousness arguments built from prior art documents are squarely within IPR; everything else falls outside it.

PGR is substantially broader. PGR allows challenge on any ground that could be raised under 35 U.S.C. section 282(b)(2) or (3), encompassing invalidity under sections 101, 102, 103, and 112, with the exception that failure to disclose the best mode is not a valid invalidity ground. Section 282(b)(2) covers invalidity on any ground specified in Part II of Title 35 as a condition for patentability; section 282(b)(3) covers failure to comply with section 112 (except best mode) or section 251. A challenger who needs to argue subject-matter ineligibility under section 101, or inadequate written description, lack of enablement, or indefiniteness under section 112, must use PGR if they want a PTAB forum for those arguments.

Which patents are covered

PGR and IPR also differ in their reach across the patent population.

Post-grant review generally applies to patents issuing from applications subject to the first-inventor-to-file provisions of the AIA. Inter partes review, by contrast, applies to any patent issued before, on, or after September 16, 2012. So when facing a pre-AIA patent issued under the first-to-invent system, inter partes review is the available PTAB trial option, while post-grant review generally does not reach it.

Institution standards

The PTAB applies different thresholds when deciding whether to institute each proceeding.

For IPR, the Board may institute upon finding a reasonable likelihood that the petitioner would prevail with respect to at least one of the claims challenged.

For PGR, the Board may institute upon finding it is more likely than not that at least one of the claims challenged is unpatentable.

Both thresholds require a credible showing against at least one claim; neither requires the petition to put all challenged claims at risk to clear the institution bar. The PGR standard is framed in terms of the probability that a claim is unpatentable, while the IPR standard focuses on the petitioner's likelihood of prevailing.

Bars to filing

Each proceeding has distinct bars that can prevent institution.

IPR carries a complaint-based bar: if the petitioner (or a real party in interest or privy of the petitioner) was served with a complaint alleging infringement and the IPR petition is filed after the applicable statutory period measured from the date of service, the petition is barred. Defendants who receive an infringement complaint must evaluate their PTAB options promptly, because delay measured from the service date can eliminate IPR entirely.

PGR has a different bar: PGR may not be instituted if the petitioner or a real party in interest filed a civil action challenging the validity of a patent claim before the date the PGR petition was filed. A counterclaim asserting invalidity does not count as such a civil action for this purpose. Unlike IPR, PGR does not carry a time bar measured from service of an infringement complaint.

Estoppel: the lasting consequence of a final written decision

Both proceedings generate substantial estoppel once the PTAB issues a final written decision.

After a final written decision in an IPR, the petitioner and its real parties in interest and privies are estopped from requesting or maintaining any USPTO proceeding, and from asserting in federal court or before the ITC, that any challenged claim is invalid on any ground that was raised or that reasonably could have been raised during the IPR. Because IPR is limited to prior art under sections 102 and 103, the estoppel's practical scope tracks that boundary.

After a final written decision in a PGR, the petitioner and its real parties in interest and privies face the same prohibition: they cannot assert invalidity on any ground raised or that reasonably could have been raised during the PGR, in USPTO proceedings, federal court, or before the ITC. Because PGR encompasses a broader range of invalidity grounds, the resulting estoppel is correspondingly broader. A petitioner who pursues PGR to a final written decision may find itself unable to raise eligibility or written-description arguments in subsequent infringement litigation.

Side-by-side comparison

Practical notes

• PGR is the PTAB path for non-prior-art grounds. Section 101 eligibility and section 112 arguments (written description, enablement, indefiniteness) are available in PGR but not in IPR. If those are your strongest invalidity arguments, assess PGR while the window is open.
• The PGR window is unforgiving. Once it closes, there is no PTAB forum for section 101 or section 112 challenges. Waiting to see how the case develops is not a viable strategy if your best grounds are non-prior-art.
• Pre-AIA patents point to IPR. Post-grant review generally applies only to first-inventor-to-file patents, so for a pre-AIA first-to-invent patent, inter partes review is the PTAB trial option.
• PGR estoppel is broad. The wider grounds available in PGR produce wider estoppel after a final written decision. Evaluate whether the comprehensive attack is worth the risk that a loss forecloses those same arguments in district court or at the ITC.
• The IPR complaint bar runs from service of the complaint, not from patent grant. The relevant clock begins when the complaint is served; a defendant who delays evaluating IPR after service risks losing that option even before the PGR window has closed.
• A PGR outcome affects a subsequent IPR. If PGR reaches a final written decision, the resulting estoppel under section 325(e) can foreclose the same grounds a follow-on IPR would rely on, since all IPR grounds are a subset of the grounds available in PGR.