The Federal Circuit recently affirmed a district court finding of noninfringement under the doctrine of equivalents (DOE) by LifeScan of a pair of patents to Pharma Tech Solutions pertaining to blood glucose monitoring systems in Pharma Tech Solutions, Inc. v. LifeScan, Inc. The Court held that prosecution history estoppel bars the infringement claims under DOE.

The patents at issue are directed towards a meter that measures blood glucose levels in a blood sample. A user places a blood sample on a test strip that has a pair of electrodes, one of which is coated with an enzyme that oxidizes glucose from the blood sample. The meter applies an electrical potential to the electrodes, measures diffusion limited current through the sample, referred to as Cottrell current, at two different times, and correlates both of these measurements with blood glucose levels.

LifeScan’s accused infringing meter measures current from two electrodes on a test strip. The final current measurements are obtained at two different time points after measurement begins. If the current difference between the final two measurements is determined to be within a defined limit, the total final current is calculated, and a single glucose result is calculated from the total final current.

Pharma Tech Solutions conceded that LifeScan’s meter does not literally infringe the claims at issue. LifeScan’s meter does not convert multiple Cottrell current readings to analyte concentration measurements, nor does it compare multiple analyte concentration measurements. Thus, both parties agreed that no literal infringement exists. Pharma Tech Solutions argued that infringement under DOE exists, however, because an analyte measurement is equivalent to a current at a given time or as a concentration.

Prosecution history estoppel arises from making a narrowing amendment to a claim during prosecution, or from surrendering claim scope through argument during prosecution. For amendment-based estoppel, US Courts generally presume that any narrowing amendment disclaims any equivalents coverage for the subject matter in between the old claim and the amended claim. A patent owner could overcome this presumption if one of three exceptions applies: (1) the equivalent was unforeseeable at the time of the application, (2) the rationale underlying the amendment bears no more than a tangential relation to the equivalent in question, or (3) there is some other reason the patentee could not reasonably be expected to have described the insubstantial substitute in question. Argument-based estoppel, on the other hand, requires clear and unmistakable surrender of subject matter, and it is measured by whether a competitor could reasonably believe that the applicant had surrendered the relevant subject matter.

Pharma Tech Solutions relied upon the second exception to overcome the disclaimer presumption of amendment-based estoppel, arguing that the claims were amended to measure current at two times, compare the currents to ensure they’re within a defined limit, and convert the current readings into glucose concentration. LifeScan asserted that the inventors amended their claims to require conversion of the Cottrell readings to analyte measurement, thereby surrendering subject matter that does not compare analyte concentration measurements. Furthermore, LifeScan asserted that the arguments presented during prosecution clearly disclaimed meters that do not perform the recited conversion and comparison steps.

During prosecution of the patents at issue, the claims were amended to recite measuring multiple Cottrell currents at different times, converting the measured Cottrell currents into analyte concentrations, and comparing the analyte concentrations. Similarly, in response to multiple Office Actions rejecting the claims as obvious, the applicant asserted that the applied art did not teach the converting and comparing steps.

The tangential relation to the equivalent in question focuses on the patentees objectively apparent reason for narrowing the amendment from the prosecution history. The Court noted that one of the claims prior to amendment would have covered any bioelectrical blood glucose monitoring system, and that the claims were subsequently narrowed to systems that convert current readings to analyte concentration measurements and compare the measurements. The Court also noted prosecution arguments emphasizing these two features to distinguish the claimed device from the applied art systems that measured and displayed diffusion limiting current. The comparison of analyte concentration measurements was deemed “integral” to the amendment and the prosecution history reflects that the converting and comparing steps were “necessary to overcome the prior art.”

Thus, the Federal Circuit did not find a tangential relation because the claims in the patent at issue were amended and clearly surrendered broader claim scope that would encompass the equivalent, and the arguments associated with the narrowing amendment emphasize that the applied art does not teach the narrowing features amended into the claim.