I would really say that it depends on the stage. Generally speaking, if it is early stage or undiagnosed, the person may be able to sign anything legally. e.g., my late FIL was in early stage of Alzheimer's back in 2004 but he was not diagnosed. He was able to sign the POA and health directive at that time. Later on, when he was in moderate stage, he could still sign but was not clear (e.g. signed the Christmas cards.) Strictly speaking, when the person is incompetent or in late moderate stage, he or she should not sign for anything legal. The legal documents may be claimed illegal if the officer finds out that the person who signs is incompetent. That is, if the officer calls the patient and he or she would not understand. That is why the POA needs to be set up early.   Regards, NC