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CO-77 Denial Code (Deactivated): Covered Days

CO-77 reported how many days of an institutional stay the payer covered. X12 deactivated it because covered days are carried in the 835's QTY segment with qualifier CA, not as an adjustment reason code.

X12 deactivated CARC77 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-77 (CARC 77)
Status: Deactivated StoppedOctober 16, 2003 (in use since January 1, 1995).
Code set: Claim Adjustment Reason Codes (CARC)
Group codes: CO (Contractual Obligation): Appeared on institutional remittances as a statistic, not as a charge reduction.
Official description: Covered days. (Handled in QTY, QTY01=CA)
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What CO-77 meant

For inpatient stays, payers need to tell the hospital how many days they recognized as covered. CARC 77 did that on older remittances. The number helped hospitals reconcile their census and benefit tracking against what the payer actually paid for.

Since a covered-day count is a quantity rather than a reason money was adjusted, it was an awkward fit for the adjustment code list.

What replaced it

No new reason code. The official text points to the QTY segment with the CA qualifier, where covered days are now reported. When some days were not covered, the dollar effect is usually explained with CO-78. Long stays paid with an outlier use CO-69, and claims that cross into an ineligible period may carry CO-239.

If you still see CO-77

It is only found in historical inpatient data. When comparing old remittances with your records, use the CO-77 value as the payer’s covered-day count and do not post it as a write-off. If covered days on a current remittance differ from your stay length, check for non-covered day adjustments and the utilization review decision before contacting the payer.

CO-78 (Non-Covered days/Room charge adjustment.): Non-covered days or room charge adjustment, the active counterpart for days not paid.
CO-69 (Day outlier amount.): Day outlier amount on long stays.
CO-239 (Claim spans eligible and ineligible periods of coverage.): The claim spans eligible and ineligible coverage periods.

CO-77 FAQ

Was CO-77 an adjustment?

Not in the usual sense. It reported a count of covered days so the hospital could match its records to the payer's, rather than explaining a dollar reduction.

How do I see covered days now?

Look at the supplemental quantity (QTY) segment on the claim in your 835 viewer. The CA qualifier identifies covered days, as the code's own text notes.

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