CO (Contractual Obligation)
Contractual Obligation
An X12 adjustment group code indicating the patient is not responsible because the amount is a contractual write-off.
1 min read · Last reviewed May 23, 2026
At a glance
- Category
- Denials & Appeals
- Acronym for
- Contractual Obligation
- Primary sources
- 2
- Workspace handoff
- denial workbench →
Where this comes up
This is denial-workbench territory. A remit posts with a CARC/RARC, the biller decides whether to rebill, appeal, or write off, and the appeal packet has to cite the chart, the order, and the payer's own policy language. Recurring patterns trace back to an upstream workflow gap.
Full definition
What it is in practice
CARC adjustments paired with group code CO reduce the allowed amount per the participation agreement — typically the difference between billed and contracted rates. These are usually correct adjustments, not denials.
How it shows up in your practice
Auto-post CO write-downs to contractual-adjustment accounts. Investigate CO-50 (non-covered services per medical necessity) and similar codes that signal a true denial.
Sources
- CMS — Claim Adjustment Reason Codeshttps://x12.org/codes/claim-adjustment-reason-codes
- CMS — 835 Electronic Remittance Advicehttps://www.cms.gov/medicare/billing/electronic-billing-edi-transactions/electronic-billing-edi-transactions-process
Filter CO denials in the Denial Workbench
Open denial workbench →Related terms
- Denials & AppealsPR (Patient Responsibility)An X12 adjustment group code indicating the patient owes the amount (deductible, coinsurance, copay, or non-covered service).
- Denials & AppealsOA (Other Adjustment)An X12 adjustment group code used when no other group code applies — frequently for coordination of benefits and other payer adjustments.
- Denials & AppealsCARC (Claim Adjustment Reason Code)X12-maintained codes communicating why a claim or service line was paid differently than billed.
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Related across the archive
- GlossaryCARC (Claim Adjustment Reason Code)X12-maintained codes communicating why a claim or service line was paid differently than billed.
- GlossaryOA (Other Adjustment)An X12 adjustment group code used when no other group code applies — frequently for coordination of benefits and other payer adjustments.
- GlossaryPR (Patient Responsibility)An X12 adjustment group code indicating the patient owes the amount (deductible, coinsurance, copay, or non-covered service).
- Glossary835 Electronic Remittance Advice (ERA)The HIPAA-mandated electronic transaction by which payers communicate payment and adjustment information to providers.
- GlossaryAppeal LetterThe written request to a payer to reconsider a denied or partially-paid claim.
- GlossaryCO-109 (Claim Not Covered by Payer)Contractual Obligation 109 — the claim is not covered by this payer/contractor.
- GlossaryCO-16 (Claim Lacks Information)Contractual Obligation 16 — the claim or service line lacks information or has submission/billing errors.
- BillingHow to Appeal an Insurance Denial: Step-by-Step GuideClaim denied? Step-by-step appeal process with payer deadlines, denial code fixes, appeal letter template, and escalation options.
This glossary entry is a research aid for billing and compliance staff. It does not provide legal, medical, or financial advice and does not replace counsel. References cited link to primary sources at HHS, OCR, CMS, eCFR, NIST, and the relevant payer or industry body.