What is the test for causation in medical malpractice cases?

Causation must be proven on the balance of probabilities. Causation has two aspects: cause in fact and cause in law. The test for cause in fact is: but for the physician’s negligence, the plaintiff would not have been injured. If this plaintiff would have suffered injury whether or not the physician was negligent, the patient cannot recover. Legal causation requires that the result must have been reasonably foreseeable. Doctors are not held responsible for unforeseeable accidents which may occur in the normal course of the exercise of their profession. The doctor is not a guarantor of the operation which he performs or the attention he gives. If he displays normal knowledge, if he gives the medical care which a competent doctor would give under identical conditions, if he prepares his patient before operation according to the rules of the art, it is difficult to sue him in damages, if by chance an accident occurs. Perfection is a standard required by law no more for a doctor than for other professional men...Cardin v. City of Montreal (1967), 1961 CanLII 77 (SCC), 29 D.L.R. (2d) 492 at p. 495.