Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.56
Fataawa Ramadhaan – Volume 2, Page 464, Fatwa No.399
Question: A man had to go to the hospital in Ramadhaan whilst he was fasting. At the hospital, they took some blood from him. Does this affect his fast?
Response: If the amount of blood they took was little, then it is not obligatory upon him to make up [the fast] for that day. However, if they took a lot [of blood], then he is to make up [the fast] for that day without doubt and also by way of precaution in relieving him of his responsibility.
And with Allaah lies all success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.