Modifier

Can anyone tell me how to code an E/M office visit to that shows the office visit is unrelated to the patient's temporary stay in a rehab facility? A patient was given IV infusion (occurs every 6 weeks) in our office. The insurance denied payment, stating the rehab facility is responsible for payment. We learned that the patient was in reahb for treatment of a fall, which has nothing to do with the IV treatments for her disease. WE are awaiting the results of our appeal, but for future reference, is there a modifier I should use? thank you