Q. Is it permitted to take a blood test on a day of fasting (some blood tests require fasting) like Tzom Gedalia, as it is similar to bloodletting, which we learned is not allowed when fasting?

A. Indeed the Talmud mentions not to do hakozas dam (bloodletting) when fasting (Shabbos 129a, Taanis 25a). Rivavos Ephraim (1:340) rules in regards to the prohibition of bloodletting on the eve of every Yom Tov (see Tur O.H. 468:3), that it does not apply to the common blood tests of today, as the amounts extracted are small and do not cause weakness or debility.

Horav Shlomo Miller’s Shlit”a opinion is that it is permitted especially at the end of the taanis, when it is close to breaking the fast.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a