A patent must include claims "particularly pointing out and distinctly claiming the subject matter which the applicant regards as his invention."  35 U.S.C. § 112 ¶ 2.  The Supreme Court construed the test for complying with this statutory requirement in Nautilus, Inc. v. Biosig Instruments, Inc., _____________:

A patent is invalid for indefiniteness if its claims, read in light of the specification delineating the patent, and the prosecution history, failed to inform, with reasonable certainty, those skilled in the art about the scope of the invention.

The claim at issue involved a heart monitor used with exercise equipment.  Detecting the heart rate is made difficult by the movement of the person doing the exercise, and the changing physiology of the body area next to the sensors.  The invention filtered out other signals to isolate the signals used to detect the heart rate.  The apparatus included two electrodes "in a spaced relationship with each other,” but included no specific distances or measurements.  The Federal Circuit allowed the claim because it was not "insolubly ambiguous" and was "amenable to construction."

The Supreme Court held the standard applied by the Federal Circuit was not sufficiently precise.  Instead, the claims must inform the person of reasonable skill in the art (in light of the patent specification and prosecution history) of the scope of the invention with reasonable certainty.  Claims that are little better than "insolubly ambiguous" would impermissibly expand the "zone of uncertainty which enterprise and experimentation may enter only at the risk of infringement claims."  QuotingUnited Carbon Co. v. Binney & Smith Co., 317 U.S. 228, 236.

Rather than determine whether the proper standard was met, the Supreme Court remanded for a determination, in the first instance, whether the claims at issue were sufficiently definite under the newly articulated test.

The new “reasonable certainty test” requires a more definite claim interpretation.  Defining the invention with “reasonable certainty” should better define the scope of the invention and enhance the ability of innovators and competitors to predict and avoid infringement.