Courts have established the "but for" test which states that but for the defendant's action, the injury would not have happened. Courts have used the "but for" rule, otherwise known as the "sine qua non" rule, to answer the question: "but for the defendant's actions, would the plaintiff have still sustained the injuries?" If the court finds that the injury would not have occurred "but for" a defendant's negligent care, which are either errors of omission or errors of commission, then the conclusion is that the defendant's actions were the proximate cause of the injury. However, just because the plaintiff can prove causation, doesn't mean that the care rendered by the defendant was below known and accepted standards. For example, if a cast was placed on a fractured lower leg, and the patient then developed a blood clot in the lung because of the cast and immobility, it doesn't mean that the doctor breached the standard of care in placing the cast, or in the way he placed the cast because blood clots can occur absent negligence, under the best of circumstances, and in the hands of the most skilled physicians. This outcome is what is known as a "maloccurrence", a bad outcome if you will, not malpractice.
As society, technology, and case law have evolved, circumstances that lend themselves to a simplistic approach to causation have lessened. Issues are seen as complex in that sometimes circumstances are viewed as contributing factors and not sole producing causes. Courts often now apportion liability between myriad defendants. Medical negligence is not as black and white as it once was. This approach is a more accurate rendering of alleged malpractice circumstance. Although it makes for a more complicated and complex litigation, it is also fairer to the involved parties.