50 vs 59 modifier

I am with a plastic surgery practice. When we bill 11970 we use modifier 50 for bilateral, but if another code is primary we bill 11970 with modifier 59. How do you know when to use a 50 or a 59?

re: 50 vs 59 modifier

50 modifier is like you said bilateral. 59 is used in situations that are usually not covered together. Example: same site, different method or a different site or session. I use it when colon polyps are removed by two different methods but at the same session.