But-For Test

The but for test is the default standard of factual causation in Ontario medical malpractice claims. The plaintiff must prove, on a balance of probabilities, that but for the defendant’s negligent act or omission the injury would not have occurred. The Supreme Court of Canada confirmed the but for test as the governing approach in Clements v Clements, 2012 SCC 32, [2012] 2 SCR 181, while preserving a narrow material contribution to risk exception for cases where but for causation cannot be established because of multiple negligent defendants.

The test does not demand scientific certainty. In Snell v Farrell, [1990] 2 SCR 311, the Court held that causation may be inferred from the evidence on a robust and pragmatic approach, and that the inference is available even without positive scientific proof, particularly where the defendant’s negligence has made it harder for the plaintiff to prove what would otherwise have happened. In medical cases the but for question is frequently decisive: a plaintiff who establishes a clear breach of the standard of care may still fail if the expert evidence cannot connect the breach to the outcome.

Posts tagged But-For Test analyze how Ontario courts have applied the but for standard and its limited exceptions across diagnosis, treatment, and informed consent fact patterns.