A doctor attends a patient. But if you're using software to bill the patient, it could be called an 'episode', though I'd choose 'service'. It really depends on what other fields you've got. For example, if it is billing software, you might have other fields for disposables, drugs, etc.
Or you could just use "Item Number". I'm assuming you mean the name of the field, not the field entries. If it's a field entry, you'd just put the item number.
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