By Arpita Bhattacharyya and Barbara McCurdy ( January 4, 2018, 1:25 PM EST) -- The inter partes review procedure includes an estoppel provision that prohibits an IPR petitioner from later raising before the U.S. Patent and Trademark Office, a district court or U.S. International Trade Commission any ground of invalidity of a claim that the petitioner "raised or reasonably could have raised" during the IPR. 35 U.S.C. § 315(e). The seemingly broad scope of the estoppel provision has concerned many petitioners about the risk of losing a patentability challenge in an IPR and being estopped from challenging the same patent claims in the district court or a subsequent IPR. Recently, the Federal Circuit, some district courts and the Patent Trial and Appeal Board have provided valuable guidance on the application of estoppel and the news is heartening for IPR petitioners. While all of these decisions have interpreted the scope of § 315(e) more narrowly than many had originally anticipated, the inconsistencies between the PTAB and the district courts' interpretations of the statute and the related Federal Circuit decisions, particularly with regard to the "raised or reasonably could have raised" provision, has continued to muddle the issue of IPR petitioner estoppel. This article discusses these conflicting interpretations and what practitioners can learn from the divergent approaches taken by the PTAB and the district courts....