My provider was on call and spent 2 dates going back to the ICU to check on a patient. The first visit was on Saturday, 99221-99223 coding, and then every 12 hours, Sunday. Would I use a initial and then a subsequent and that is it? I know I cannot bill the 2 visits separate that were done on Sunday. Is there a modifier I am missing? The only modifier I found is a 24 but that has specifics of post operative. Thanks.