I'm wondering why can't one take a blood pressure or draw blood on the arm of the side that a mastectomy was done?

Breast ca sx: After breast ca, sx there is changes in blood flow drainage especially if there was axillary nodes removed that will inter fer with lymphatic drainage . You will be at high risk for phlebitis and lymphangitis.

Arm swelling: There is lymphatic interruption with radical mastectomy -- so frequent and prolonged constriction with BP cuff may cause swelling and pain in the affected arm and hand.