Whoever has Blood taken from him while he is fasting
What is the ruling on whoever has blood taken from him while he is fasting during Ramadhan? The reason that this is done is for a blood test, and the blood is taken from his right arm. The amount that is taken is a moderate amount (in a syringe).
The likes of this blood test do not corrupt the fast. Rather, it is pardoned because it is from that which there is a need for. It is not from the things that are known from the pure Islamic law to break the fast.
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Page no. 275 Vol: 2