On March 3, 1997, the U.S. Supreme Court issued its decision in Warner-Jenkinson v. Hilton Davis. In its unanimous (9-0) decision, the Court adhered to the doctrine of equivalents but reversed the decision of the Court of Appeals for the Federal Circuit reported in our November 1995 Newsletter (N.S. 186) and remanded the case to the Federal Circuit for further fact finding.

Hilton Davis Chemical Co. sued Warner-Jenkinson Co., Inc. for infringement of its U.S. Patent 4,560,746 ('746 Patent). The '746 Patent claims a process for purification of dyes using ultrafiltration. The elements of the main claim that were in issue were the pore diameter of the filtration membrane used (5-15 angstroms); the hydrostatic pressure used during filtration (approximately 200 to 400 p.s.i.g.) and the pH at which the ultrafiltration took place (from approximately 6.0 to 9.0). The allegedly infringing process was found to operate at pressures in the range of 200 to nearly 500 p.s.i.g. and a pH of 5. At the trial, there was testimony that the pore diameter of the filters used by the alleged infringer would have to be 5 to 15 angstroms. At trial, it was held that Warner-Jenkinson's ultrafiltration process to purify dyes conducted at a pH of 5 did not literally infringe the claims but did infringe under the doctrine of equivalents. In a 7-5 en banc decision, the Court of Appeals for the Federal Circuit affirmed the decision of the trial court.

Warner-Jenkinson filed a writ of certiorari to the U.S. Supreme Court. One of the questions before the Supreme Court was whether "a product or process that does not literally infringe upon the express terms of a patent claim may nonetheless be found to infringe if there is 'equivalence' between the elements of the accused product or process and the claimed elements of the patented invention". The Supreme Court reaffirmed the doctrine of equivalents by providing that it is to be applied by determining whether the accused product or process contains elements identical or equivalent to each claimed element of the invention rather than viewing the accused product or process as a whole.

Another issue addressed by the Supreme Court was the effect of the doctrine of prosecution history estoppel on the doctrine of equivalents. The former doctrine precludes a patent owner in an infringement suit from obtaining a construction of a claim that resurrects subject matter surrendered during prosecution of the application. In the past, the doctrine of prosecution history estoppel was applied when the examiner rejected a claim as unpatentable over the prior art and in response, the applicant canceled or amended the claim in order to distinguish from the art to obtain issuance of a patent. The Supreme Court did not limit its discussion of prosecution history estoppel to claim amendments made to distinguish from the prior art. The Supreme Court stated that the burden is to be placed on the patentee "to establish the reason for an amendment required during patent prosecution. The court then would decide whether that reason is sufficient to overcome prosecution history estoppel as a bar to application of the doctrine of equivalents to the element added by amendment. Where no explanation is established, however, the court should presume that the PTO had a substantial reason related to patentability for including the limiting element added by amendment." This presumption is, however, rebuttable. One concern that is apparent initially is that, while the doctrine of prosecution history estoppel has traditionally been based on claim amendments made to distinguish from the prior art, the Supreme Court seems to be taking this a step further and applying the doctrine of prosecution history to any claim amendments needed to establish patentability such as those required to clarify a claim. Further commentary on this issue is expected to be made by the Federal Circuit.

The Supreme Court noted that during prosecution of the '746 Patent, the claim was amended to add the phrase "at a pH from approximately 6.0 to 9.0". The upper limit was added to distinguish over a reference that disclosed an ultrafiltration process that was carried out at a pH above 9.0. The record was not clear on why the lower limit was added and thus it was not clear whether prosecution history estoppel could apply to this amendment. The Supreme Court remanded the case to the Federal Circuit to "consider whether reasons for that portion of the amendment were offered or not and whether further opportunity to establish such reasons would be proper."

The Supreme Court left for the Federal Circuit to decide what test is to be applied to determine whether there is infringement under the doctrine of equivalents. One test that can be used is derived from the traditional test set out in the 1950 Supreme Court case Graver Tank & Mfg. Co. v. Linde Air Products Co. This test focuses on the function served by a particular claim element, the way that element serves that function and the result obtained from that element. Another test is whether the differences between the elements are insubstantial. The Supreme Court noted that the proper time for evaluating equivalence of the elements and knowledge of interchangeability between the elements is at the time of infringement, not at the time the patent issued.

The Federal Circuit had stated that infringement under the doctrine of equivalents is a question of fact to be submitted to a jury in a jury trial and to be decided by the judge in a bench trial. The Supreme Court declined to consider whether the determination of infringement under the doctrine of equivalents was a task for the judge or the jury since resolution of that question was not necessary for the Supreme Court to reach a decision in this case.

Upon return of the case to the Federal Circuit, that court in turn remanded the case back to the district court. The district court was instructed to apply the Supreme Court's holding that amendments to claims made during prosecution should be rebuttably presumed to relate to issues of patentability if the record does not show any other reason. As noted above, the record did not establish why there had been an amendment to the lower end of the pH range in the claim and the patentee was entitled to be given the opportunity to provide an explanation so that the trial court could determine whether that explanation was sufficient to overcome the presumption that it was made to establish patentability. The Court noted that it expected that "the PTO and applicants will henceforth usually include in the prosecution history express statements of their reasons for requiring or making claim changes or interpretive assertions" but that for older cases, the district courts should use their discretion to determine how best to reach a decision on why an amendment was made.