Prosecution History Can Be Used During Claim Construction

April 3, 2012

John S. Harmon PhD

What This Means To You

  • Do not refer to “the present invention” during prosecution.
  • Be careful of creating ambiguous arguments that may, or may not, disavow embodiments.
  • Claim constructions should not contradict a disclosed embodiment unless there has been a clear disavowal of that embodiment.


In Advanced Fiber Tech. Trust. v. J&L Fiber Serv., Inc., the US Court of Appeals for the Federal Circuit confirmed a patent’s prosecution history may be properly used to construe terms and limit the scope of a patent.

Case Background

In September 2003, Advanced Fiber Tech. Trust (AFT) submitted for reissue a patent directed to screening devices for paper production. After receiving a Notice of Allowance in December 2007, AFT sued the defendant alleging patent infringement. Disclosed embodiments included solid filtering plates with perforations and filtering plates made from interwoven wires. During reissue, the claims were rejected in view of a patent showing identical screens for a radial flow catalytic converter.

In response to the rejection, AFT argued the disclosed slot widths of the catalytic converter were three times the size of the “present invention.” It also maintained the term “screen” was defined in the Handbook of Pulp and Paper Technology as a perforated barrier and it argued the screens disclosed in the prior art were not intended for treating paper pulp.

During claim construction, the district court adopted AFT’s argument and limited the slot widths to one third of those in the prior art. Ignoring the wire screen embodiment, the district court also construed the non-claim term of “perforated” as being pierced or punctured.

Decision Analysis

The Federal Circuit held it was proper to limit the size of the slots to one third of the prior art since there had been a clear disavowal in the prosecution history. The court appears to have emphasized the use of the language “the present invention” from the prosecution history to support this finding.

In contrast, the Federal Circuit found it was improper to construe the non-claim term of “perforation” because non-claim terms are not usually construed. The court also found the district court’s claim construction ignored the wire screen embodiment which had not been disavowed.


Be careful in presenting arguments in prosecution since you may wind up limiting claim coverage.

Using phrases such as “the present invention” during prosecution are more likely to result in those arguments limiting the claims.

This case also highlights the importance of not inadvertently disavowing embodiments by introducing possible definitions that might be construed as precluding an embodiment. Similarly, proposed claim constructions should not contradict a disclosed embodiment unless there has been a clear disavowal during the prosecution history.