What is the test for determining whether a professional who adheres to common professional practice is liable for negligence in a medical malpractice case?

Moreover, courts have the discretion to assess liability even in the face of uncontradicted evidence of common professional practice. As L’Heureux-Dubé J. stated in Roberge v. Bolduc, 1991 CanLII 83 (SCC), [1991] 1 S.C.R. 374, at p. 437: The fact that a professional has followed the practice of his or her peers may be strong evidence of reasonable and diligent conduct, but it is not determinative. If the practice is not in accordance with the general standards of liability, i.e., that one must act in a reasonable manner, then the professional who adheres to such a practice can be found liable, depending on the facts of each case. [Emphasis in original.]