The burden of proof for infringement under the doctrine of equivalents still is a preponderance of the evidence even when an alleged equivalent is claimed in a separate patent.

The district court found that the alleged infringer infringed under the doctrine of equivalents a patent related to medical imaging technology, specifically positron emission tomography (“PET”). A majority of the Federal Circuit affirmed.

The alleged infringer argued that in cases where an alleged equivalent is separately patented, a heightened evidentiary burden should be required: Clear and convincing evidence rather than the standard preponderance of the evidence should be used for showing infringement under certain circumstances. In particular, the alleged infringer argued that because a finding of equivalence “constructively invalidated” another patent that contained the allegedly equivalent feature, the district court legally erred by failing to instruct the jury that equivalence must be found by clear and convincing evidence (i.e., the evidentiary burden required to overcome the statutory presumption of validity).

Though it found the alleged infringer’s arguments “interesting,” the majority declined to adopt the proposed new evidentiary standard. Rather, where, as here, the alleged equivalent is claimed in a separate patent, this fact, when weighed together with all other relevant evidence, may make equivalency “considerably more difficult to make out” by a preponderance of the evidence. Moreover, even if equivalence were relevant to obviousness, the jury properly found infringement by only a preponderance of the evidence, which could not constructively invalidate the other patent. Furthermore, equivalence and obviousness are analyzed under completely different legal frameworks: An invalidity analysis requires a comparison between the prior art and the claimed “subject matter as a whole, not separate pieces of the claim,” and equivalence is analyzed at the time of infringement whereas obviousness is analyzed at the time the patent was issued.

In dissent, Judge Prost explained that she would adopt the following test for these situations: a separately patented (and presumptively non-obvious) substitution cannot be “insubstantial” unless some fact distinguishes the equivalence finding from the Patent Office’s earlier non-obviousness determination in issuing the patent. And a finding that, from the time of the separately patented invention to the time of accused infringement, the state of the art advanced in such a way as to make a previously non-obvious substitution obvious, would allow an equivalence finding without upsetting the presumption of non-obviousness.