As long as the person is careful not to swallow the blood, which is coming out of the mouth without their consent, bleeding that gets on the teeth has no effect on them. It is not considered to be breaking the fast in this instance, so the person who tests the blood is not required to make up his fasting day. Similar to the above, if a person has nosebleeds and is aware that he does not swallow it, he is under no obligation to make up that fasting day. Sheikh Muhammad bin Uthaymeen.