Answered by Shaykh Faraz Rabbani
Question : Would a blood test (having blood drawn through a needle) invalidate my fast?
Answer: No, a blood test does not invalidate the fast, as it is merely the taking out of blood. It is only disliked if it could weaken one from being able to fast.
The fast is invalidated when anything enters the body cavity, which is why injections and drips (even those that have nutritive value) do not invalidate the fast. [Bada’i` and Masa’il al-Siyam]