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CO-A3 Denial Code (Deactivated): MSP Liability Met

CO-A3 meant Medicare's liability as secondary payer had been met, so no further Medicare payment was due. X12 deactivated it without naming a replacement.

X12 deactivated CARCA3 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-A3 (CARC A3)
Status: Deactivated StoppedOctober 16, 2003 (in use since January 1, 1995).
Code set: Claim Adjustment Reason Codes (CARC)
Group codes: CO (Contractual Obligation): The provider could not collect more from Medicare; any balance followed the primary payer's and Medicare's rules.
Official description: Medicare Secondary Payer liability met.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What CO-A3 meant

When Medicare is secondary under Medicare Secondary Payer rules, it calculates a secondary payment based on what the primary insurer paid. CARC A3 told the provider that this calculation came out at zero: the primary’s payment had already satisfied what Medicare would be responsible for.

What replaced it

X12 did not name a replacement. Current Medicare secondary remittances show the effect of the primary’s processing with OA-23. If Medicare believes another payer is primary but it is missing from the claim, you may see OA-22. A zero secondary payment is otherwise evident from the payment amounts themselves.

If you still see CO-A3

Only older Medicare remittances should carry it. When reconciling, match it to the primary’s payment. The remaining balance, if any, should normally be adjusted off according to Medicare’s secondary payment rules rather than billed to the patient in full. For current claims, verify that the primary’s allowed and paid amounts were reported correctly, because errors there change Medicare’s secondary calculation.

OA-23 (The impact of prior payer(s) adjudication including payments and/or adjustments. (Use only with Group Code OA)): Impact of prior payer adjudication, the usual code on Medicare secondary claims.
OA-22 (This care may be covered by another payer per coordination of benefits.): This care may be covered by another payer per coordination of benefits.
CO-94 (Processed in Excess of charges.): Processed in excess of charges.

CO-A3 FAQ

What does it mean for MSP liability to be met?

When Medicare pays second, its payment is limited by what the primary paid. If the primary's payment already reached the amount Medicare would allow, Medicare owes nothing more.

Can I bill the patient after CO-A3?

Usually only for amounts Medicare's secondary rules leave to the patient, if any. When the primary's payment met Medicare's limit, the patient often owes little or nothing.

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