(a) A health care agency shall be in writing and signed by the principal or by some other person in the principal's presence and by the principal's express direction. A health care agency shall be attested and subscribed in the presence of the principal by two or more competent witnesses who are at least 18 years of age. In addition, if at the time a health care agency is executed the principal is a patient in a hospital or skilled nursing facility, the health care agency shall also be attested and subscribed in the presence of the principal by the principal's attending physician.

(b) No health care provider may act as agent under a health care agency if he or she is directly or indirectly involved in the health care rendered to the patient under the health care agency.

(c) An agent under a health care agency shall not have the authority to make a particular health care decision different from or contrary to the patient's decision, if any, if the patient is able to understand the general nature of the health care procedure being consented to or refused, as determined by the patient's attending physician based on such physician's good faith judgment.