Court of Appeal rules in Smith & Nephew v ConvaTec on the construction of a numerically defined patent claim

9 minute read
03 July 2015

In a dispute between Smith & Nephew and ConvaTec, the Court of Appeal has ruled that "between 1% and 25%" extends to all values ≥ 0.5% and <25.5%. overturning the first instance decision of mr justice birss, court appeal rejected a construction based on significant figures, which would have born no result to distribution random error in practice.

The Court of Appeal's judgment in Smith & Nephew v ConvaTec [2015] EWCA Civ 607 was handed down on 24 June 2015.


ConvaTec's patent EP(UK) number 1,343,510 (the 'Patent') claims a method of preparing a light stabilized antimicrobial material "characterised in that the method comprises...the steps of:...(c) subjecting said polymer, during or after step (b) to one or more agents...the agent being present in a concentration between 1% and 25% of the total volume of the treatment".

In 2013, Smith & Nephew began proceedings seeking a declaration of non-infringement. Their 'Modified Process' comprised the steps of the patented method save that the concentration of the 'binding' agent was no more than 0.77%.

ConvaTec counterclaimed that Smith & Nephew's Modified Process infringed the Patent, and also that an earlier process (the 'Original Process') infringed. The data generated from carrying out the Original Process were used in support of Smith & Nephew's application for marketing authorisation for a wound dressing. The concentration of binding agent used in the Original Process ranged from 0.93% to 0.97%.

In November 2013, Birss J ruled that in the patent in suit, "between 1% and 25%" meant "greater than or equal to 0.95% and less than 25.5%". He reached this conclusion after adopting the "significant figures" approach to numerical rounding. The result was that Smith & Nephew's Modified Process did not infringe the patent but its Original Process did. Both parties appealed.

Construction - the law

In the Court of Appeal Lord Justice Kitchin gave the leading judgment.

The interpretation of a patent claim was considered by the House of Lords in Kirin Amgen v Hoechst Marion Roussel [2004] UKHL 46. In Virgin Atlantic Airways v Premium Aircraft Interiors [2009] EWCA Civ 1062 Jacob LJ summarised the principles with emerged from Lord Hoffmann's speech in Kirin Amgen.

To this summary Kitchin LJ added the following principles as drawn from Lord Hoffmann's speech and having a bearing on the present case:

"First, the reader comes to the specification with the benefit of the common general knowledge and on the assumption that its purpose is to describe and demarcate an invention. Second, the patentee is likely to have chosen the words appearing in the claim with the benefit of skilled advice and, in so far as he has cast his claim in specific rather than general terms, is likely to have done so deliberately."

Kitchin LJ also endorsed the reasoning of Mr Peter Prescott QC, sitting as a deputy judge of the High Court, in Auchinloss v Agricultural & Veterinary Supplies [1997] RPC 649. After noting that the concept of an 'immaterial variant' stems from the use of descriptive words or phrases, the deputy judge stated as follows:

"... Where the patentee has expressed himself in terms of a descriptive word or phrase there may be room for supposing that he was using language figuratively, and did not intend to restrict himself to the purely literal meaning. But where the patentee has defined an integer of his claim in terms of a range with specified numerical limits at each end, his purpose must be taken to have been to claim thus far and no further. His reason for doing so may not be apparent, but it may exist all the same, for instance it may lie 'buried in the prior art'. Further, in this case I believe that there are evident reasons of convenience and certainty which would have led him to claim in this way, as I have observed."

Kitchin LJ noted that it is 'standard practice' for scientists deliberately to express numerical values to a particular degree of accuracy. As a result, the degree of precision with which any particular number is written conveys to the reader how the author intended the number to be understood. While the approach to be adopted to the interpretation of claims containing a numerical range is no different from that to be adopted in relation to any other claim, analysis of the authorities on numerical claim construction (UK and EPO) reveals the following additional points:

  1. The scope of any such claim must be exactly the same whether one is considering infringement or validity.
  2. There can be no justification for using rounding or any other kind of approximation to change the disclosure of the prior art or to modify the alleged infringement.
  3. The meaning and scope of a numerical range in a patent claim must be ascertained in light of the common general knowledge and in the context of the specification as a whole.
  4. It may be the case that, in light of the common general knowledge and the teaching of the specification, the skilled person would understand that the patentee has chosen to express the numerals in the claim to a particular but limited degree of precision and so intends the claim to include all values which fall within the claimed range when stated with the same degree of precision.
  5. Whether that is so or not will depend upon all the circumstances, including the number of decimal places or significant figures to which the numerals in the claim appear to have been expressed.

Construction - the patent

In the present case, it was not suggested that the numerical range in the claim had been used figuratively or descriptively. The issue was rather whether the skilled person would have understood the figures defining the range to have been expressed to a particular degree of exactitude.

Smith & Nephew's primary position was that the figures were exact; any concentration of binding agent less than 1.0% or more than 25.0% being outside the scope of the claim. This contention was rejected by the Court of Appeal, as it had been by Arnold J:

  • The word "between" merely denoted that 1% and 25% were the outer limits of the range, without itself saying anything about the degree of precision with which those limits were expressed. It was apparent from the specification of the patent that the patentee had well in mind the possibility of expressing numerical values with a high degree of precision. Examples of precision ranged from zero decimal places to two decimal places.
  • Against this background, the skilled person would understand that the patentee had chosen to express the numerical limits of the range in the claim to only a limited degree of accuracy.
  • Also supporting this conclusion was the teaching of the specification that the binding agent concentration was not critical in the method the invention.

Smith & Nephew's secondary position was to support Arnold J's finding on construction. The 'significant figures' approach to rounding is a little complex where the lower limit is 1. This is because 0.5% is already expressed to an accuracy of one significant figure, as is 0.9%. Therefore the lowest value that rounds to 1% when expressed to an accuracy of one significant figure is 0.95%. The consequence of this is asymmetry in the error margins at the outer limits of the range.

In contrast, rounding to the nearest whole number extends the range to ≥ 0.5% and <25.5%, which has a much larger relative error margin at the lower end of the range.

Was the significant figures approach justified? The Court of Appeal ruled that it was not. The approach produced a result that bore no relationship to the distribution of random error in practice. There was no reason to suppose that the accuracy with which the concentration of binding agent needed to be determined varied depending upon where the bottom of the range was. The expert evidence indicated that it was not the number of significant figures that was important but the precision with which the number was written. The skilled person would understand the claimed range to have been expressed to the nearest whole number.


The Court of Appeal's judgment was a fairly narrow and academic one, but an important one for patentees and practitioners to be aware of in the context of numerically defined claims.

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