What is the law of causation in medical malpractice cases?

Speaking for the majority of the court, Rouleau J.A. reviewed the development of the law of causation from Snell v. Farrell forward in time. Specifically, he contrasted application of the “but for” test to the “material contribution” test with the former obliging a plaintiff in a delayed medical diagnosis case to prove, on a balance of probabilities, that the delay caused or contributed to the unfavourable outcome. As such, the plaintiff fails if he cannot establish that the outcome could have been avoided with prompt diagnosis and treatment.