An invention must have “utility” - that is, it must be “useful” - to be patentable. An patent that purports to claim an invention that is not useful is invalid. Whether something is considered to be “useful” in patent law appears to depend on the patent specification. If a patent promises a specific result, then the invention must fulfill that promise to have utility. If the patent is silent on the usefulness of the invention, then a “mere scintilla” of utility will suffice for validity.
The word “useful” appears in the definition of the word “invention” under s. 2 of the Patent Act:
“invention” means any new and useful art, process, machine, manufacture or composition of matter, or any new and useful improvement in any art, process, machine, manufacture or composition of matter1
There is a helpful discussion in Halsbury's Laws of England, (3rd ed.), vol. 29, at p. 59, on the meaning of “not useful” in patent law. It means “that the invention will not work, either in the sense that it will not operate at all or, more broadly, that it will not do what the specification promises that it will do”. There is no suggestion here that the invention will not give the result promised. The discussion in Halsbury's Laws of England, ibid., continues:.. the practical usefulness of the invention does not matter, nor does its commercial utility, unless the specification promises commercial utility, nor does it matter whether the invention is of any real benefit to the public, or particularly suitable for the purposes suggested. [Footnotes omitted.]
and concludes:. . it is sufficient utility to support a patent that the invention gives either a new article, or a better article, or a cheaper article, or affords the public a useful choice. [Footnotes omitted.]
Canadian law is to the same effect. In Rodi & Wienenberger A.G. v. Metalliflex Limited4, (affirmed in this Court,  S.C.R. 117) the Quebec Court of Appeal adopted at p. 53 the following quotation from the case of Unifloc Reagents, Ld. v. Newstead Colliery, Ld.5 at p. 184:If when used in accordance with the directions contained in the specification the promised results are obtained, the invention is useful in the sense in which that term is used in patent law. The question to be asked is whether, if you do what the specification tells you to do, you can make or do the thing which the specification says that you can make or do.
The Court thus appeared to distinguish between two different levels of utility. This dual utility requirement has gained importance in recent years. A more modern statement of the utility requirement is the decision of the Federal Court of Appeal in Eli Lilly Canada Inc v Novopharm Ltd (Olanzapine)6:
Where the specification does not promise a specific result, no particular level of utility is required; a “mere scintilla” of utility will suffice. However, where the specification sets out an explicit “promise”, utility will be measured against that promise. The question is whether the invention does what the patent promises it will do.
The “mere scintilla” standard of utility has been a long-standing feature of Anglo-Canadian patent law.7 This is a low standard, and is not meant as a tool to judge the merits of the invention. Thus, considerations of practical or commercial usefulness are irrelevant. Merely providing a choice to the public is sufficient to meet a “mere scintilla” of utility.8
However, one limit to the minimum standard of utility is that the invention must actually work (or be capable of working). Thus, if the invention is inherently inoperable for the purpose for which it was designed (such as in the case of a purported perpetual motion machine), it lacks utility.9
Nonetheless, there has been a historical tendency to adjudicate utility of the invention against its “intended purpose”. Thus, in Northern Electric Co Ltd v Brown's Theatre Ltd,10 Maclean P. wrote:
An invention to be patentable must confer on the public a benefit. Utility as predicated of inventions means industrial value. No patent can be granted for a worthless art or arrangement. Here there is described and claimed something that lacks utility because it is inoperable for the purpose for which it was designed.
And in the trial-level decision of Wandscheer et al v Sicard Limitée,11 the Court stated:
The test of utility of an invention is that it should do what it is intended to do and that it be practically useful at the time when the patent is issued for the purpose indicated by the patentee.
If a patentee promises a certain result, utility will be measured against the promise rather than the mere scintilla standard.12 There is a distinction between mere statements of advantages or goals on the one hand, and promises of utility on the other hand.13 Promises must be explicit, as understood from the perspective of the skilled person.14
In Apotex Inc v Pfizer Canada Inc (Celecoxib), the Federal Court of Appeal summarized the jurisprudence on promised utility in the following way:15
 The promise doctrine will hold an inventor to an elevated standard only where a clear and unambiguous promise has been made. Where the validity of a patent is challenged on the basis of an alleged unfulfilled promise, the patent will be construed in favour of the patentee where it can reasonably be read by the skilled person as excluding this promise. This approach can be traced back to the earliest mentions of the promise doctrine. In Consolboard, the source of the promise doctrine in Canadian law, the Supreme Court of Canada reiterated the longstanding principle that (Consolboard at 521, citing Western Electric Company, Incorporated, and Northern Electric Company v. Baldwin International Radio of Canada,  S.C.R. 574 at 570):… where the language of the specification, upon a reasonable view of it, can be so read as to afford the inventor protection for that which he has actually in good faith invented, the court, as a rule, will endeavour to give effect to that construction.
 This rule in favour of saving an invention rather than invalidating it in case of ambiguity has been consistently applied by this Court. While the principle is sometimes invoked by reference to the original language found in Consolboard (Anastrozole at paras. 17 and 19) affirming AstraZeneca Canada Inc. v. Mylan Pharmaceuticals ULC, 2011 FC 1023,  F.C.J. No. 1262 at para. 88), it is at other times given effect through the requirement that promises be “explicit” (see Olanzapine at para. 76, Eli Lilly and Company v. Teva Canada Limited, 2011 FCA 220,  F.C.J. No. 1028 at paras. 18 to 21 [Atomexetine], Plavix FCA at para. 49). Drawing an analogy with the threshold test applicable to selection patents, the Court in Plavix FCA expressed the need for explicitness by saying that a promise must be supported by language “… at least as clear and unambiguous as that used to establish the advantages of the selection over the compounds of a genus patent” (Plavix FCA at para. 66). It follows that it is not enough to merely label a promise as “explicit” if it can only be supported on the basis of equivocal inferences and ambiguous indications (Plavix FCA at paras. 64-66).
At the date of patent filing, the utility of the invention (whether promised or mere scintilla) must be either have been demonstrated or capable of being soundly predicted based on the information and scientific knowledge available to the skilled person.16
Utility is demonstrated by “evidence that establishes that the embodiment at issue does in fact work in a manner that gives rise to the advantages stated in the patent.”17 Test results that are “strongly suggestive” of utility with “no other logical explanation” are sufficient to demonstrate utility.18 Test results need not be “conclusive”.19 It is not necessary to test every minute variation to establish utility.20
However, it is sometimes difficult to meet the test for demonstrating utility, particularly for pharmaceutical patents. For example, in Novopharm Ltd v Eli Lilly & Co,21 the Federal Court held that a three week clinical trial was not sufficient to demonstrate clinical utility of atomoxetine for treatment of ADHD. In Pfizer Canada Inc v Canada (Minister of Health),22 the Federal Court of Appeal held that “single dose” studies on animals and healthy humans did not demonstrate utility of latanoprost for treatment of glaucoma.
Where utility is not demonstrated at the filing date, it may still be proven by the doctrine of sound prediction. This rationale for this doctrine was laid out by the Supreme Court in Apotex Inc v Wellcome Foundation Ltd (AZT):23
The doctrine of “sound prediction” balances the public interest in early disclosure of new and useful inventions, even before their utility has been verified by tests (which in the case of pharmaceutical products may take years) and the public interest in avoiding cluttering the public domain with useless patents, and granting monopoly rights in exchange for misinformation.
There are three components to the doctrine of sound prediction (which are questions of fact to be determined by the trial judge):
The line of reasoning must be a prima facie reasonable inference of utility, or be a reasonable theory or hypothesis.25
There has been some judicial debate regarding the third element of sound prediction, namely, what is required to be disclosed in the patent to support a finding of sound prediction. Generally, courts have required both the factual basis and sound line of reasoning to be set out in the disclosure of the patent itself.26 This disclosure, as with other interpretations of the patent, are to be read with the common general knowledge of the skilled person.27 This disclosure requirement has been subject to challenge, and it has been suggested by the Supreme Court (in obiter) that the prevailing interpretation may be incorrect.28 It has also been suggested that the disclosure requirement only applies to “new use of old compound” type patents.29