screening colonoscopy CPT code

Medicare Patient has an order for an initial screening colonoscopy because they are 50. during the pre op, patient states to the Dr. that she has been having diarrhea and constipation. He does the procedure and finds nothing.

I know the coding will be 1. V76.51 because that was the reason for the procedure initially, BUt would this be a G0121....or would this need to be a 45378 since the patient had diagnostic issues when she came for that colon?

Maybe the V76.51 shouldn't be used as primary since they came with these new sxs? But I am more concerned about the correct CPT.

Thanks

screening colonoscopy CPT code