Question: A man had to go to the hospital during the month of Ramadaan while he was fasting. When he was there, they took blood from him. Does this invalidate his Sawm (Fast)?
Answer: If the amount of blood taken from the person is customarily considered a small amount, then it is not obligatory on him to make up for that day. However, if the amount of blood taken from him is customarily considered a large amount, then he should make up for that day to avoid an area of scholarly disagreement and to be on the safe side through discharging one’s responsibility… read more here.