ASC Coding guidelines for dx
Documentation to support the ASC coding guidelines that you requested, are in the ICD9 code book under coding guidelines, Section IV, Diagnostic Coding and Reporting Guidelines for OP services, under the letter "O".
It states:
For ambulatory surgery, code the diagnosis for which the surgery was performed. IF the POSTOPERATIVE dx is known to be different from the preoperative dx at the time the diagnosis is confirmed, SELECT THE POSTOPERATIVE DX for coding since it is the most definitive.
I know every payor is different, but I would suggest you make sure you get something in writing from the payor to support doing it differently. I would also check with your audit company to find out what rules they were following when they dinged you.
Hope this helps
Mary