Q. One that has to do periodically blood tests when fasting and finds it hard to do so, can a blood test be done on Tisha Beav since he is fasting anyway?
A. On question 219 regarding a blood test on Tzom Gedalia we wrote; “The Talmud mentions not to do hakozas dam (bloodletting) when fasting (Shabbos 129a, Taanis 25a). However, 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.”
On question 2494 regardin a blood test on Assara Beteveth that according to some opinions is more severe than others, we wrote: “In our case, if there is a need for someone who finds it difficult to abstain another ten hours or more from eating or drinking coffee for a blood test, the Rov maintains it is permitted. As far as the possibility of a taanis being a detrimental omen day, the Rov quoted the rule often used in similar matters; “Man delo kopid, lo kopdinon leih” or one that is not himself concerned, we are not concerned for him. (Pesachim 110b).
Regarding Tisha Beav Horav Shlomo Miller’s opinion is similar.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a