= If the blood comes out as a result of a person's action or choice and its
amount is large and has an effect on the person's body, then this invalidates
his fast (such as donating blood). One should make up that day (so as to avoid
an area of scholarly dispute and be on the safe side and be sure that he has
fulfilled his obligation).
= If the blood comes out as a result of a person's actions or choice and its
amount is small and it does not harm the person, then it does not invalidate the
fast, such as the small amount of blood that is taken for testing.
= If the blood comes out as a result of no deliberate action, such as an
accident or nosebleed or an injury to any part of the body, the fast is still
valid even if the amount of blood is large.
= If the blood that comes out as a result of no deliberate action and is of such
a large amount that the person becomes weak and unable to fast, then it is
permissible for him to break his fast and he should fast another day to make up
= The blood of menstruation (haidh) and post-natal bleeding (nifas) invalidates
the fast for the women, she must make up for that day and rest of the days until
she is pure again.
See, Fatawa Islamiyyah, 2/132 and al-Lajnah al-Daa'imah li'l-Buhooth al-'Ilmiyyah