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CO-79 Denial Code (Deactivated): Cost Report Days

CO-79 reported cost report days, a figure institutional providers use when preparing cost reports. X12 deactivated it because the value is carried in the 835's inpatient adjudication segment (MIA15).

X12 deactivated CARC79 on October 16, 2003. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Code: CO-79 (CARC 79)
Status: Deactivated StoppedOctober 16, 2003 (in use since January 1, 1995).
Code set: Claim Adjustment Reason Codes (CARC)
Group codes: CO (Contractual Obligation): Reported on institutional remittances as data for cost reporting, not as a charge reduction.
Official description: Cost Report days. (Handled in MIA15)
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What CO-79 meant

Hospitals and other facilities that file Medicare cost reports need day counts that match the payer’s processing. CARC 79 delivered the number of cost report days for a claim on the remittance. It did not explain a reduction; it passed along data the facility’s reimbursement team would use later.

What replaced it

No CARC replaced it. According to the official text, the value is handled in MIA15, part of the Inpatient Adjudication Information segment that carries Medicare-specific inpatient data. That segment also holds other institutional figures that used to be squeezed into adjustment codes.

Active institutional codes that still appear as adjustments include CO-A5, CO-74, and CO-76.

If you still see CO-79

It exists only in historical inpatient remittances. Reimbursement analysts reviewing old cost report years may still encounter it in archived files. Treat it as a count of days, never as dollars to post. For current years, pull cost report days from the MIA segment and reconcile them against your internal census and the payer’s reports.

CO-A5 (Medicare Claim PPS Capital Cost Outlier Amount.): Medicare PPS capital cost outlier amount, another institutional reporting item.
CO-74 (Indirect Medical Education Adjustment.): Indirect medical education adjustment on institutional claims.
CO-76 (Disproportionate Share Adjustment.): Disproportionate share adjustment.

CO-79 FAQ

What are cost report days used for?

Facilities that file cost reports with Medicare track days associated with covered stays. The payer's count helps the facility reconcile its cost report figures with what was processed.

Where does this number appear now?

In the Inpatient Adjudication Information (MIA) segment of the 835. The official text of the code points to element MIA15.

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