Even though some jurisdictions still retain common law contractual concepts in dealing with medical malpractice issues, medical malpractice is now generally considered to be an independent action in tort, rather than in contract. In the same sense that the ordinary body of negligence law defines negligence as the doing or the failure to do something that a person of ordinary prudence would or would not do under the same or similar circumstances, the law of medical malpractice defines medical negligence as the doing or the failure to do something that a reasonably prudent health care professional in that field would or would not do under the same or similar circumstances. On the other hand, the law holds even medical professionals to certain minimum requirements of care, and evidence that few people in a given medical field exercise caution in an area where caution should be exercised would not preclude a finding of medical negligence. In medical malpractice cases the plaintiff must establish through expert testimonythe standard of care required of professionals in the field of the defendant and that the defendant breached or failed to adhere to that standard of care, thereby causing the plaintiff's injury. A negative result in medical treatment in and of itself does not mean that the defendant committed malpractice.
Medical treatment carries with it no guarantee of a successful outcome.
In many medical procedures there are risks which cannot be avoided even if the health care professional exercises the greatest caution. These are called unavoidable risks. On the other hand, even risks which are unavoidable even when the greatest care has been exercised, may in a particular case, be shown to have resulted from lack of due care by the health care professional. The standard legal defenses of contributory neglicence and assumption of the risk are generally considered to be applicable in medical malpractice cases, although by the very nature of the superior knowledge of the health care professional over that of the patient, there are probably less instances where these defenses can be effective. In terms of factual defenses, health care professionals raise a number of arguments in opposing malpractice claims, several of which have little merit, but all of which create significant obstacles to a plaintiff.