Litigating Reduction to Practice: Traps for the Unwary

The difference between winning and losing a billion dollars in a patent case can be as seemingly insignificant as a date. But when that date is a disputed priority date for the asserted patent, it is not only potentially outcome-determinative, but also quite difficult to establish. A priority dispute often arises in patent litigation when the accused infringer asserts a prior art reference that predates the filing of the application for the patent-in-suit, but postdates the time at which work on the patented invention began. Faced with this art, the patentee has two options: fight the prior art on the patent's merits, potentially limiting the scope of the claims and impairing its infringement arguments, or 'swear behind' the reference by establishing a pre-filing priority date before the asserted reference, and thus eliminate the reference as prior art. In most cases, swearing behind the reference is the better option because it does not typically affect the substance of the claims. By swearing behind the reference, the patentee does not have to draw distinctions between the reference and the claims that can be used against it later in claim construction or in the context of a noninfringement argument. But establishing an earlier priority date can be tricky because it generally requires evidence, including corroborating documents, showing that the invention was reduced to practice before the inventor filed the patent application.