I have a blood test appointment in two days. Can I have the blood test whilst fasting? Will it break my fast? Is it permissible to have a blood test whilst fasting?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
It is permissible to have a blood test whilst fasting. However, it may weaken a person. Therefore, it may be advisable to postpone the blood test for after Ramadhan if it is not urgent.
And Allah Ta’āla Knows Best
Mufti Faraz ibn Adam al-Mahmudi,
 ثُمَّ خُرُوجُ الدَّمِ مِنْ الْبَدَنِ لَا يُفَوِّتُ رُكْنَ الصَّوْمِ وَلَا يَحْصُلُ بِهِ اقْتِضَاءُ الشَّهْوَةِ وَبَقَاءُ الْعِبَادَةِ بِبَقَاءِ رُكْنِهَا (المبسوط للسرخسي ج 3 ص 57 دار المعرفة)
This answer was collected from DarulFiqh.com, which is operated under the supervision of Mufti Faraz ibn Adam al-Mahmudi, the student of world renowned Mufti Ebrahim Desai (Hafizahullah).