Incident-to billing is done under the physician's NPI. It's important to remember that incident-to is for follow-up visits only, and it can't be for a new problem. If the patient hasn't been seen by the physician before or has a new problem, the billing is then done under the nurse practitioner's NPI.
In our infectious disease practice, we provide in-office antibiotic infusion therapy so we bill drug and infusion codes, and the patients do sometimes see the physician on the same day as an infusion. In these instances, the E&M charge is separately payable as long as a full visit was completed and documented. A 25 modifier must be billed with the E&M charge.
Someone who does chemo billing can correct me if chemo rules are different from infusion (insurance carriers have often compared our infusion billing to chemo when it comes to questions like this), but you should be able to bill the E&M with a 25 modifier. Note - A 99211 cannot be billed, with or without a 25 modifier, if on the same DOS as the infusion (chemo). It will be denied as content to the infusion (chemo).
Hope this helps.