Step-by-step guide
You will need to navigate to the lab setup screen, to the report setup tab. Here you can update the subject lines for any/all of the types of communications that leave your system. The fax subject line is down a few in the list. In any of those fields, there is an add icon to the left that will allow you to insert data from the record (such as the accession number) into the fields to the right. Clicking on the little icon will produce a pop-up that will allow you to insert a field from the system.
Generally, the field you choose to insert will paste at the end of the line, so when you type in Test results for Accession #, and then hit the icon, choose the accession number field, and that should put the information where you need it.