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CO-A4 Denial Code (Deactivated): PPS Capital Day Outlier

CO-A4 reported the Medicare inpatient prospective payment system capital day outlier amount. X12 deactivated it without naming a replacement; capital and outlier amounts now appear in other codes and in the 835's MIA segment.

X12 deactivated CARCA4 on April 1, 2008. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Quick facts

Code
CO-A4 (CARC A4)
Status
Deactivated StoppedApril 1, 2008 (in use since January 1, 1995).
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): An institutional payment component, never a patient liability.
Official description
Medicare Claim PPS Capital Day Outlier Amount.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-A4 meant

CARC A4 belonged to a set of Medicare inpatient codes that reported pieces of the hospital’s payment. This one carried the capital portion of a day outlier: extra payment tied to the number of days in an unusually long stay, attributed to the capital component of the prospective payment system.

What replaced it

X12 did not name a successor. The capital cost outlier still has its own active code, CO-A5. Other outlier amounts use CO-69 for day outliers and CO-70 for cost outliers. The 835’s Inpatient Adjudication Information segment also carries PPS capital amounts.

If you still see CO-A4

It should only appear in archived Medicare inpatient remittances. Hospital reimbursement staff reviewing old stays can use it to reconstruct how an outlier payment was split. Post it as part of the payment calculation, not as a write-off or denial.

  • CO-A5 (Medicare Claim PPS Capital Cost Outlier Amount.): Medicare Claim PPS capital cost outlier amount, the still-active capital outlier code.
  • CO-69 (Day outlier amount.): Day outlier amount.
  • CO-70 (Cost outlier - Adjustment to compensate for additional costs.): Cost outlier adjustment.

CO-A4 FAQ

What was a capital day outlier?

Medicare's inpatient payment includes a capital component. Outlier payments for unusually long stays could include a capital portion, which CO-A4 reported.

Why might this code have become unnecessary?

The official text does not give a reason. Medicare's inpatient outlier policy has changed over time, and the 835 has dedicated inpatient fields for capital amounts.