Praise be to Allah.
“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 the blood 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.” (Fatawa al-Lajnah al-Daimah, 10/263)
Shaykh ‘Abd al-‘Aziz ibn Baz (may Allah have mercy on him) was asked about taking blood in Ramadan for the purpose of testing.
“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 Shari`ah to invalidate the fast.” (Fatawa Islamiyyah, 2/133)
Shaykh Ibn Jibrin 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.” (Fatawa Islamiyyah, 2/133)
And Allah knows best.