What is the test for a plaintiff to bring an action for delayed medical diagnosis and treatment?

The Ontario Court of Appeal explained this principle further in Cottrelle v. Gerrard, in the following terms: …[I]n an action for delayed medical diagnosis and treatment, a plaintiff must prove on a balance of probabilities that the delay caused or contributed to the unfavourable outcome. In other words, if, on a balance of probabilities, the plaintiff fails to prove that the unfavourable outcome would have been avoided with prompt diagnosis and treatment, then the plaintiff's claim must fail. It is not sufficient to prove that adequate diagnosis and treatment would have afforded a chance of avoiding the unfavourable outcome unless that chance surpasses the threshold of “more likely than not.”[16]