One key trade-off for petitioners challenging the validity of a patent at the U.S. Patent and Trademark Office (“PTO”) is being estopped from making a parallel challenge in district court. If the PTO issues a final written decision in an inter partes review (“IPR”), the petitioner is estopped from later urging in court that a reviewed patent “claim is invalid on any ground that the petitioner raised or reasonably could have raised” in its IPR. 35 U.S.C. § 315(e)(2). However, the estoppel statutory provision does not state precisely when prior art that was not used in an IPR will be barred from use in court as part of invalidity grounds that “reasonably could have [been] raised” in the IPR.

On March 18, 2016, the U.S. District Court for the Northern District of Illinois issued an opinion in Clearlamp, LLC v. LKQ Corp.,  holding that the “reasonably could have raised” language covers any prior art that “could have been found by a skilled searcher’s diligent search.” Acknowledging the lack of precedent on the issue, the Court largely based its interpretation on legislative history—in particular, on a similar statement made by then-Senator Jon Kyl (R-Az.) during debate in the U.S. Senate.

Applying its newly articulated standard to the case before it, the Court determined both that the plaintiff had the burden of showing that a skilled searcher would have found the prior art in question (a particular datasheet) during a diligent search and that the plaintiff had not met this burden. Consequently, the defendant was not estopped from using the prior art. The Court noted that one way the plaintiff could have made the required showing would have been “(1) to identify the search string and search source that would identify the allegedly unavailable prior art and (2) [to] present evidence, likely expert testimony, why such criterion would be part of a skilled searcher’s diligent search.”

The Clearlamp decision raises some notable practice points. First, it remains very important to ensure that pre-IPR prior art searches are thorough and complete. Second, it is not a foregone conclusion that defendants will be estopped from using all patent- and non-patent publication-type references in district court after an IPR. Instead, plaintiffs bear the burden of establishing that estoppel should apply because a diligent search would have discovered the newly asserted reference, while defendants may attempt to avoid estoppel with evidence demonstrating why a diligent search would not have uncovered the new reference before the IPR was filed.