Donating Blood While Fasting
Question:Is donating blood during the day in Ramadan permissible or does it break the fast?
A:If he donated blood and a lot was taken, then certainly this invalidates the fast because it is analogous to cupping, and that would be to draw blood from his vein in order to help the sick or for safe keeping in case of emergency.
As for (drawing) a little such as taking with a syringe for analysis and examination (purposes), then this does not break the fast.
– Sheikh Ibn Jibreen, Fatawa Ramadan - Volume 2, Pages 466-467, Fatwa No.403.