As far as Islamic Shari`ah is concerned, there is nothing wrong in doing a blood test while fasting, and it does not break one’s fast. If there is a medical necessity to draw blood from a fasting person and because drawing much blood, one becomes over exhausted and was forced to break the fast, there will be nothing wrong with that and he or she can make up for that day later.
Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto,Ontario, Canada, stated: 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.