What is the test for establishing causation in medical malpractice cases?

The leading case on the question of causation in Canada now is Athey v. Leonati et al., 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458; 203 N.R. 36; 81 B.C.A.C. 243; 132 W.A.C. 243; 140 D.L.R.(4th) 235; 31 C.C.L.T.(2d) 113; [1997] 1 W.W.R. 97. This case basically restates the law of causation as it has stood for many years. Mr. Justice Major, speaking for the court at p. 466 [S.C.R.] said: "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: … "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;"