How does estoppel on the merits work in patent interference cases?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

Estoppel on the merits in patent interference cases prevents a losing party from seeking claims that are anticipated or obvious based on the subject matter of the lost count. According to MPEP 2308.03:

“A losing party is barred on the merits from seeking a claim that would have been anticipated or rendered obvious by the subject matter of the lost count.”

This means that if an applicant loses on priority for a specific subject matter, they cannot later claim that same subject matter or obvious variations of it. For example, if an applicant loses on a count drawn to subject matter X, they cannot file a continuing application with a claim to subject matter X or a claim that would be obvious in view of X.