How low can hemoglobin go before you need a transfusion?

It depends... What's the cause? How quickly has the hemoglobin dropped/is there any ongoing bleeding? Is there evidence of inadequate tissue perfusion? What other acute & chronic medical problems does the patient have? In critically ill patients who aren't bleeding, a hemoglobin as low as 7 g/dl is generally well tolerated. Decision to transfuse should be based on desired physiologic effect, not on a number.