While performing a “second pass” peer review for a health plan, we were asked to review the coding and DRG assignments of both the hospital and the health plan’s DRG auditing vendor, a large nationally known company. Our findings: The provider assigned DRG 286, CIRCULATORY DISORDERS EXCEPT AMI, W CARD CATH W MCC. The Auditing Vendor changed the DRG to 287, CIRCULATORY DISORDERS EXCEPT AMI, W CARD CATH W/O MCC by removing a Major Complication/Comorbidity code (MCC) of acute kidney failure with tubular necrosis. The focus on the MCC was based on their audit selection algorithms. However, when we conducted a thorough and complete audit of all coding elements and documentation, we discovered that the principal diagnosis was not correct and had not been identified by either party. The patient’s elective surgery had been cancelled after admission when the patient was found to be in kidney failure, however the principal diagnosis was related to a complication of a procedure that had not taken place as planned. This was missed by both parties who were focusing only on the MCC code Our correction to DRG 683, RENAL FAILURE WITH CC reduced the original DRG by over 60%, an additional 25% savings over the work of the primary Audit Vendor.