What is the "but for" test in medical negligence cases?

The "but for" test is used as the standard for establishing causation in most negligence cases. [See Note 4 below] The test is well summarized at paras. 13-14 of Athey v. Leonati: Causation is established where the plaintiff proves to the civil standard on a balance of probabilities that the defendant caused or contributed to the injury. [See Note 5 below] The general, but not conclusive, test for causation is the "but for" test, which requires the plaintiff to show that the injury would not have occurred but for the negligence of the defendant. (Citations omitted) [page293] Thus, the "but for" approach in an action for delayed medical diagnosis and treatment obliges a plaintiff to 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". (Cottrelle, at para. 25) (b) The "material contribution" test