Q. Since one is required to fast when taking some types of blood tests, can one do it during a taanit such as Assara Beteveth that according to some opinions is more severe than others.
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 1566 regarding being more stringent to fast on Asarah Beteves for someone who due to age and weakness does not fast besides Yom Kipur and Tisha Beav, we wrote:
“Beis Yosef (O.H. 550) quotes Avudraham’s opinion that if Asarah Beteves was to occur on Shabbos we would fast on that day, since it is similar to Yom Kippur in which the pasuk mentions “beetzem hayom haze” (Yechezkel 24).
Toras Chaim (550: 4), wonders why Bais Yosef would mention the above detail since it does not have a practical difference, as this fast day cannot according to our luach coincide with Shabbos. He explains that it may be for the purpose of being more stringent on this taanis than others and other prohibitions, such as washing also apply. He mentions that his own father, the Machane Chaim, on the end of his life, when he was very weak and was not supposed to fast, he forced himself to do so on Asarah Beteves.
Biur Halocho (551: 1) mentions that some maintain in Asarah Beteves the stringencies of the Nine Days.
Ratz Katzvi (16) maintains that on this taanis a baal nefesh should begin fasting from the night before. Weddings should therefore be avoided on that night.
Bnei Isoschor (Teves14) explains that this fast day marks the very beginning of the churban and all the days of avelus that we maintain, and is therefore more severe and strict.
However, in practice many Poskim maintain that one does not have to be more rigorous than in other fast days (Shulchan Aruch and Mishna Berura 550:1, Maharam Mibrisk 3: 29, Nitei Gavriel – Chanuka 60: 3 et. al.)
Horav Shlomo Miller’s Shlit’a opinion is similar.”
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).
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a