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CO-99 Denial Code (Deactivated): MSP Adjustment Amount

CO-99 reported the adjustment Medicare made when it paid as secondary under Medicare Secondary Payer rules. X12 deactivated it without naming a replacement; prior payer impact is now reported with CARC 23.

X12 deactivated CARC99 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-99 (CARC 99)
Status: Deactivated StoppedOctober 16, 2003 (in use since January 1, 1995).
Code set: Claim Adjustment Reason Codes (CARC)
Group codes: CO (Contractual Obligation): The MSP reduction was not billable to the patient beyond their normal cost-sharing.
Official description: Medicare Secondary Payer Adjustment Amount.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What CO-99 meant

When Medicare is the secondary payer, its payment depends on what the primary insurer paid and allowed. CARC 99 showed the resulting Medicare Secondary Payer adjustment on the remittance, so the provider could see why Medicare’s payment was lower than its normal allowance.

What replaced it

X12 did not name a successor. On current Medicare secondary claims, the effect of the primary’s processing is reported with OA-23. If Medicare believes another payer should be primary and it is not on the claim, you may see OA-22. When the primary’s coverage rules were not followed, OA-136 applies.

If you still see CO-99

It appears in historical Medicare remittances. When reconciling, match the amount against the primary payer’s remittance for the same service. Any difference should be explained by the primary’s payment and adjustments.

For current MSP claims, make sure the primary’s payment details are included when billing Medicare, and keep the patient’s MSP questionnaire up to date so the order of payers is correct.

OA-23 (The impact of prior payer(s) adjudication including payments and/or adjustments. (Use only with Group Code OA)): The impact of prior payer adjudication, used when Medicare pays after another payer.
OA-22 (This care may be covered by another payer per coordination of benefits.): The care may be covered by another payer per coordination of benefits.
OA-136 (Failure to follow prior payer's coverage rules. (Use only with Group Code OA)): Failure to follow the prior payer's coverage rules.

CO-99 FAQ

What is Medicare Secondary Payer?

MSP rules make another insurer, such as an employer group health plan, workers' compensation, or liability insurance, pay before Medicare in certain situations. Medicare then pays secondary, if anything.

How do I handle an old CO-99 amount?

Reconcile it against the primary payer's payment. The adjustment reflects the part of the charge already handled by the primary, not a loss to write off without review.

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