I stumbled across this article in Rosen's Emergency Medicine Textbook. Pretty good explanation from a very reliable source.
on page 2598:
Unconscious or Medically Incompetent Adult
In an emergency, if the Jehovahs Witnesss beliefs are unknown, physicians may transfuse the patient because consent will be implied under the emergency doctrine. It is irrelevant if the spouse, mother, or other family members adamantly refuse to allow the transfusion for religious reasons. The states compelling interest in preserving life out- weighs the familys expression of the patients religious preferences.118
In the past, when a Jehovahs Witnesss beliefs and transfusion preferences were known in advance, but the patient was incompetent at the time of the emergency, the courts tended to support transfusion until the patient became competent and could refuse transfusion contemporaneously.112,124 The modern trend is to accept objective evidence of the patients wishesfor example, a signed card carried by the patient that identifies him as a member of the Jehovahs Witnesses and sets out his religious objection to blood transfusion. The card may be accepted as adequate evidence of the patients intent, like a form of advanced directive, which is binding on hospitals and physicians. In at least six states, if the card is dated and signed before two witnesses, it is statutorily valid.125 Even if the blood refusal card does not conform to a states advance directive statute, it should be considered strong evidence, but not necessarily determinative, of the Jehovahs Witnesss wishes. Advance directives are merely a means to express an individuals rights and are not the exclusive means to express those rights legally.118,119 Jehovahs Witnesses increasingly use state statutorily defined advance directive methods to legally express their intentions.126 Emergency physicians should, however, be certain the card or advance directive actually belongs to the patient.
Of interest, no Jehovahs Witness has successfully sued a health care provider to recover damages in cases in which blood was withheld on the basis of an apparently valid blood refusal card. Also, criminal, civil, or professional misconduct liability has never been imposed on health care providers for forgoing treatment the patient did not want.127