I always used the 59 modifier when I coded for the facility side. I used the 51 for the physician side. Sometimes I still use a 59 even on physicians, it just depends. But I never used the 51 on the facility at all, in fact, our encoder would flag us and tell us, you can't do that!!! And as stated in the previous post, The modifiers in front of CPT book tells you it's not on the approved list for Hospital outpatient use.