This excerpt is taken from the book of Fadhilat al-Shaykh Mubashir Ahmad Rabbani, may Allah have mercy on him, Ahkam and Masail of Ramadan.
Ruling on donating blood while fasting?
Question:.
Is it permissible to donate blood while fasting during the month of Ramadan, or does it break the fast?
Fatwa:.
When a person donates blood and a significant amount of blood is drawn from their body, this act breaks the fast and is analogous to cupping, where a considerable amount of blood is drawn from the veins to save a patient or for emergency situations. However, if a small amount of blood is drawn, it does not break the fast, as is the case when a small quantity of blood is drawn through a syringe for analysis and tests in laboratories.