Intellectual Property Law

New Matter SUMMER 2015 Volume 40, Number 2

Runaway jurisprudence: Has the "But For" Test for Proving Inequitable Conduct in Patent Cases Gone Awry, Gone Rogue, or Gone Quiet?

GINO CHENG Winston & Strawn LLP

On May 25, 2011, the Federal Circuit set forth in its en banc Therasense opinion a new, heightened test requiring that a prior art reference, knowingly withheld by the applicant, be "but for" material to the patent’s issuance in order to constitute inequitable conduct that would render the obtained patent unenforceable.1 On January 8, 2015, the Federal Circuit denied the combined petition for panel rehearing and for rehearing en banc2 in its decision in American Calcar, Inc. v. American Honda Motor Co., Inc. (hereinafter "American Calcar IT’).3 The inequitable conduct claims at issue had been evaluated previously by the court in June of 20114 (hereinafter "American Calcar I"), on the heels of the Therasense decision, and had been remanded to the district court for more detailed determinations of materiality and intent. With the original Therasense majority currently occupying merely a third of the seats5 and with judge Prost, a member of the dissent, assuming the duties of chief judge, does American Calcar II break rank with post- Therasense precedent—including American Calcar I—and offer any clues as to what direction the court may be headed?


In the United States, a patent applicant owes a duty of candor and good faith to the U.S. Patent and Trademark Office (PTO).6 This duty may be breached whenever the applicant fails to disclose known material prior art references prior to the issuance of its patent.7 The Theasense court ruled 6-1-48 in favor of the adoption of a new, heightened standard for deciding the materiality of references withheld by an applicant seeking to deceive the patent office—a "but for" test.9 In other words, a reference is material "if it would have blocked patent issuance under the PTO’s evidentiary standards."10 An applicant’s failure to disclose such references, if coupled with a specific intent to deceive the PTO, could lead to every claim in the offending patent being held unenforceable.11 The majority also tempered its test with an exception for instances of egregious misconduct, which would be categorically material.12

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