Answer
Wa `alaykum as-salam wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
There is nothing wrong in doing a blood test while fasting, and it does not break one’s fast.
In responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
There is nothing wrong in going for a blood test while fasting. Taking blood does not break your fast in any way; if, however, drawing too much blood makes you weak and therefore you are afraid that you may end up being forced to break your fast because of exhaustion, then you are better advised to avoid it if you can while fasting.
If, however, there is a medical necessity to do so, then you should go ahead, even if you are forced to break the fast because of over exhaustion; you can make up for it later.
Allah Almighty knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.