As a beginner I started to code labs. There were many cases when on the same form were marked both a comprehensive metabolic panel (80053) that includes total protein(84155) and protein electrophoresis with the same 84155. The software promted me to add a modifier 59 or 91 in order to proceed. I thought that 91 is inappropriate because it seemed to be the same sample done for different markers by different technology and not a subsequential test for the same marker, and therefore I added 59. However, during one of the last seminars at work we were told that it should be 91 . Please help me with my confusion. If I really was wrong putting 59 for a long period of time did I cause a lot of damage in terms of overbilling and what should be done to fix it(all of it happened 5 yr ago).