Hi Michelle,
I've had a couple of these cases recently. One was due to neoplasia and needed to be put down. The other appeared to be due to Hemobartonella (or whatever it's called these days - Mycoplasma?) despite the fact that the cat was FeLV and FIV negative. Both cats received transfusions. The Hemobart cat also received doxycycline and Gui Pi Tang, and made a full recovery.
So whether to transfuse depends on the degree of compromise. If the heart is going over time and the cat anorexic, then a transfusion is advisable. If on the other hand, the anemia is so chronic that the cat is actually quite functional and not too tachycardic, then maybe herbs (and doxy?) will be enough. In general, though, it's been Gui Pi Tang in severely anemic cats that has brought them back from the brink for me. Or else its sub-formula - Dang Gui Bu Xue Tang, which is a 5:1 mix of Huang Qi (Astragalus) to Dang Gui (Angelica).
Hopefully that's useful advice. I know there is a specialized test that can check for Mycoplasma these days as well. To my mind, doxycycline appears so well tolerated that I have few qualms about using it at least for a couple of weeks in almost any type of patient.
Steve