Does taking blood invalidate the fast?
If the blood that is taken from him is regarded as being a small amount, then he does not have to make up that day. But if it is considered to be a large amount, then he should make up that day so as to avoid an area of scholarly dispute and so as to be on the safe side and be sure that he has fulfilled his duty.
Fataawa al-Lajnah al-Daa’imah, 10/263
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) was asked about taking blood in Ramadaan for the purpose of testing. He replied:
Such tests do not invalidate the fast, rather they are forgiven because they are something that is necessary and they are not like the things that are known in sharee’ah to invalidate the fast.
Fataawa Islamiyyah, 2/133
Shaykh Ibn Jibreen said:
If a person donates blood and a large amount is taken, that invalidates his fast, by analogy with cupping, which refers to the act of letting blood flow from the veins in order to relieve sickness or so that this blood may be stored for emergencies. But if it is a small amount then that does not invalidate the fast, such as taking a small amount with a needle for the purpose of testing.
Fataawa Islamiyyah, 2/133.