Posted on 29/6/2015

Pick a number between 1 and 25: claim construction – Smith and Nephew v Convatec

A patent infringement case comes down to the numbers.

As many readers will be aware, the most important part of a patent application is its claims: the claims define, in words, what the invention is.

Some claims include ranges, limiting the scope of protection to the extent that certain components are present in an amount falling within the range.  So it was with ConvaTec’s European Patent No. 1343510.  This patent claimed a process for silverisation of gel-forming fibres used in wound dressings, which involved subjecting the gel-forming fibres to an agent, such as a chloride salt, which facilitates the binding of the silver to the polymers.

The relevant patent claim required, among other features, that the binding agent must be present in a concentration of “between 1% and 25% of the total volume of treatment”.

Smith and Nephew developed a process including all of the other claimed features, but in which the concentration of binding agent was no more than 0.77% and asked the court to declare that as such, the method did not infringe the patent; 0.77% being outside the range of 1–25%.  Smith and Nephew argued that 1–25% meant exactly that, i.e. a range between precisely 1% and precisely 25%.

Needless to say, ConvaTec disagreed and argued that values outside these precise boundaries would fall within them, since anything above 0.5% can be rounded up to 1% and anything below 25.5% can be rounded down to 25%.  Thus, in effect, ConvaTec argued that 1-25% meant 0.5-25.5%. 

First Instance Decision

The High Court Judge, Birss J, took the view that the reasonable interpretation was to look at the numbers in terms of significant figures; as such he decided that the range was between 0.95% and 25.5%. Accordingly, the decision was that Smith and Nephew’s method did not infringe.

ConvaTec, unsurprisingly did not like the result and appealed against it, Smith and Nephew too disagreed, arguing in favour of their original approach.

The Appeal Decision

The Court of Appeal heard from both parties and in the end agreed with ConvaTec that the range had been expressed in terms of the nearest whole number.  Accordingly, for any amount above 0.5% the nearest whole number was 1 and therefore a concentration of 0.77% fell within the scope of the claim.


A number of criticisms have been made of the appeal decision, mainly arguing that from the point of view of a chemist, who would be carrying out the process, 0.5% is a long way from 1%. Most chemists when presented with a 0.5% solution, having asked for a 1% solution, would be pretty unimpressed, whereas 0.95% would normally be considered close enough to 1%.  The decision goes to show how shades of grey can be found even in features that look black and white.

Contact us

If you need more information or are considering obtaining patent protection, please contact us and one of our patent attorneys will be able to discuss this with you.

Wilson Gunn