Question: What is the ruling regarding someone who has half a measure of blood drawn out of his right arm for analysis purposes whilst fasting during the day in Ramadhaan?
Response: The fast is not nullified for this (analysis), rather it is exempted from. This is because it is a necessity and not from that which is known, from the pure Sharee’ah, to break the fast.
Shaykh Ibn Baaz
Fataawa Ramadhaan – Volume 2, Page 465, Fatwa No.400. fatwaonline.com