Answered by Shaykh Faraz Rabbani
Would a blood test nullify a fast. e.g. if someone would insert a needle and take some blood from me by means of it?
No, a blood test does not invalidate the fast, as it is merely the taking out of blood.
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]