When do I use medical billing modifier 59? This is a great question. It is one that many don’t ask, but most don’t know the correct answer to. One of the most important things to know about the medical billing modifier 59 is which code on which to append it. There are some basic medical billing rules that can teach you which code to use with modifier 59. The general assumption about modifier 59 (Distinct procedural service) is that it should be linked to the lower-valued code of the pair. Although this may be true a lot of times, it is not always true. There is a much better rule …