First off newhall94, the last sentence in your first post? Forget about it. We all want to make it better but we have to code correctly. It is fraud if you knowingly change a code to get a higher reimbursment or to make it easier on the pt. I know that wasn't your intention by typing that (I hope), but it's a thought we can't have. We have to enter the codes that are correct according to the documentation.
Now, on to your question. Technically, you could use the G codes (G0121-average risk, or
G0105-high risk). They are HCPCS codes for Medicare, but some payors will reimburse for them.
Now you say he presented to you for a straight colonoscopy and there were no findings. Did he present for a screening? Or did he have symptoms (abd pain, diarrhea, etc)?
Onto the 33 modifier. 45378 is a diagnostic procedure code, so again, techinically, you could use the 33 modifier but you shouldn't have to. You did code the procedure correctly the first time and it sounds like you will need to call Anthem BCBS.