CO-26 Denial Code: Expenses Incurred Before Coverage Started

CO-26 is a Claim Adjustment Reason Code that means the payer’s eligibility records show the patient’s coverage hadn’t started yet on the date of service. It’s an eligibility denial, not a coding or medical necessity problem, and the CO group code means the payer has put the amount on the provider. (Primary sources: X12 Claim Adjustment Reason Codes · WPS, Common Claim Denials · 42 CFR 435.915)

Quick answer: CO-26 means the payer says the patient’s coverage hadn’t started on the date of service. Check the effective date for that exact date. If coverage was retroactive or the payer’s date is wrong, ask for reprocessing with proof. If another plan covered that date, bill it. Otherwise, follow your contract before billing the patient.

Part of our denial codes library. Written by Manikandan, a revenue cycle management specialist with more than 10 years of US medical billing experience. Last reviewed: 7 October 2026. Official code wording is quoted word for word from X12; Medicare rules link to their CMS or Medicare contractor source.

CO-26 denial code description (official X12 wording)

X12 Claim Adjustment Reason Code 26, verbatim:

“Expenses incurred prior to coverage.”

Source: X12 Claim Adjustment Reason Codes, checked 7 October 2026. Start: 01/01/1995. No usage note.

The wording is short, so the key word is prior. The payer isn’t saying the service is never covered. It’s saying the patient wasn’t covered yet when it happened. X12 once had a broader code for this, CARC 28 (“Coverage not in effect at the time the service was provided.”), but stopped it on 10/16/2003 with the note “Redundant to codes 26&27.” So today a “no coverage on this date” denial is split in two: 26 for before coverage, 27 for after it ended.

What CO-26 means in plain English

Each part of the code answers a different question:

• CO (Contractual Obligation) is the payer’s classification of who carries the amount. CMS says CO is used “when a contractual agreement between the payer and payee, or a regulatory requirement, resulted in an adjustment. Generally, these adjustments are considered a write off for the provider and are not billed to the patient” (Medicare Claims Processing Manual, Ch. 22, §60.1).
• CARC 26 tells you why: the date of service on the claim falls before the coverage effective date the payer has on file.

Common causes include a new job’s plan that starts on the first of the next month, a newborn billed before the baby was added to a parent’s plan, a Medicaid application that hasn’t been approved yet, and a Medicare patient seen before their Part B start date. The payer may also simply have the wrong start date.

CO-26 vs CO-27, CO-31 and CO-109

These codes all look like “eligibility problems”, but each one points to a different fix:

Code	Official X12 text	What it tells you
26	“Expenses incurred prior to coverage.”	The patient is known to the payer, but coverage started after the date of service
27	“Expenses incurred after coverage terminated.”	Coverage existed but had ended before the date of service. See CO-27
31	“Patient cannot be identified as our insured.”	The payer can’t match the patient at all. See CO-31
109	“Claim/service not covered by this payer/contractor. You must send the claim/service to the correct payer/contractor.”	Wrong payer or contractor. See CO-109

If the payer found the member but the dates don’t line up, that’s 26 or 27. If it couldn’t find the member, that’s 31.

CO-26 vs PR-26 vs OA-26 vs PI-26

CARC 26 can come with any group code, and the group code changes who the payer says owes the money:

Group + 26	Who carries the amount	Can you bill the patient?	When you’ll typically see it
CO-26	Provider	Not under the payer’s classification. Check your contract first	Commercial and managed care plans you’re contracted with
PR-26	Patient	Yes, the payer assigned it to the patient	WPS lists Medicare entitlement denials as “CO/PR 26”
OA-26	Neither group fits	Check the remark codes and your contract	Used “when no other group code applies” (CMS Ch. 22, §60.1)
PI-26	Payer-initiated reduction	No	Non-Medicare remits only. CMS lists just CO, OA and PR as valid on Medicare remittances

A CO-26 is a strange case: if the patient truly had no coverage, there’s no member contract protecting them. Even so, don’t send a bill until you’ve confirmed there’s no retroactive coverage and checked what your participation agreement says about non-members.

Remark codes you may see with CARC 26

WPS lists Medicare’s CO/PR 26 with “Remark Code: None”, so many Medicare remits carry no RARC at all. Other payers may add one of these:

RARC	Official X12 text	What it adds
N30	“Patient ineligible for this service.”	A general eligibility flag
N52	“Patient not enrolled in the billing provider’s managed care plan on the date of service.”	The managed care enrollment didn’t cover that date
N687	“Alert: This reversal is due to a retroactive disenrollment.”	A paid claim was taken back after the payer changed its enrollment dates

RARC wording is quoted from the X12 RARC list, checked 7 October 2026. Pairings vary by payer, so treat these as clues and confirm with an eligibility check.

Where CO-26 shows up in the 835

Here’s a trimmed, illustrative example for a $210.00 office visit line:

SVC*HC:99204*210.00*0.00**1~
DTM*472*20260728~
CAS*CO*26*210.00~

• CAS*CO*26*210.00 is the group code, reason code and adjusted amount.
• DTM*472 is the date of service. Compare it with the coverage effective date in the payer’s eligibility response.
• There’s no LQ remark segment here, which is common for this code.

How to fix CO-26: decision path

1. Run eligibility for the exact date of service. Use the payer portal or a 270/271 inquiry. For Medicare, CMS’s HIPAA Eligibility Transaction System lets you “check Medicare beneficiary eligibility data in real-time” (CMS HETS).
2. Compare the effective date with the date of service. Note the plan, member ID and start date the payer returns. Check that the date of service on the claim is right, too. A typo in the year or month can trigger CO-26.
3. Look for retroactive coverage. For example, Medicaid must make eligibility effective “no later than the third month before the month of application” for someone who received covered services and would have been eligible (42 CFR 435.915). Group health plans must start a newborn’s special-enrollment coverage “on the date of birth” (45 CFR 146.117(b)). Once the payer loads that coverage, ask it to reprocess or resubmit the claim.
4. Look for the plan that did cover that date. If the patient had a prior plan, Medicaid or other coverage on the date of service, bill that payer. If the claim was simply sent to the wrong payer, see CO-109.
5. If the payer’s start date is wrong, dispute it with proof. Send an enrollment confirmation, ID card with the effective date, or employer or exchange letter, and ask for the eligibility record to be corrected and the claim reprocessed.
6. If no coverage existed, close it out correctly. Follow the group code and your contract. For Medicare, WPS says that if the date of service is before the entitlement date, “bill the patient or their appropriate insurance for the services in question.”

Medicare and major payer notes

Medicare Part B entitlement. WPS explains CO/PR 26 on Medicare claims this way: “Our records show that the patient did not have Part B coverage on the date of service billed.” Its fix is to “Check the effective date for Part B entitlement on the patient’s Medicare card and on WPS SNAP.” WPS adds that “Medicare Part B entitlement and termination is determined by the Social Security Administration (SSA)”, and if the patient’s date and the MAC’s date differ, “the beneficiary should contact 1-800-MEDICARE for assistance” (WPS, Common Claim Denials).

Medicare Advantage. If the patient was in a Medicare Advantage plan on the date of service, that plan, not the MAC, is the payer. Original Medicare will usually deny with a different code. See CO-24.

Medicaid. Retroactive eligibility can cover up to three months before the application month under 42 CFR 435.915(a). The rule also says “The State plan must specify the date on which eligibility will be made effective”, so check your state’s Medicaid manual for how to bill retro periods.

Newborns on a parent’s group plan. A plan must give at least 30 days after the birth to request enrollment, and coverage must begin on the date of birth (45 CFR 146.117(b)). A CO-26 on a baby’s first claims often means the enrollment hasn’t been processed yet.

Commercial plans. Plans set their own start dates, such as the first of the month after hire. Read the eligibility response, not just the ID card.

Worked example (illustrative)

A patient starts a new job and is told her employer plan begins August 1, 2026. She comes in on July 28, 2026, shows the new card, and the visit is billed to the new plan. The remit comes back CAS*CO*26*210.00.

Reading it: the eligibility response shows an effective date of 08/01/2026. The new plan is right: the visit was four days before coverage started.

Fix: the patient says her Marketplace plan ran through July 31. A 271 check confirms it. The front desk adds that plan as primary for July 28, and the claim is billed to it. The new employer plan stays on file for August onward.

Prevention: when a patient shows a new card, ask for the start date and keep the old coverage until the new plan’s effective date.

CO-26 reprocessing request template

Use this when coverage was in place on the date of service (for example, after a retroactive enrollment) or when the payer has the wrong start date. Replace the bracketed text.

[Date]
[Payer name] Provider Disputes / Reconsiderations
[Address or portal reference]

Re: Request to reprocess claim denied CO-26 (expenses incurred
prior to coverage)
Patient: [Name]   Member ID: [ID]   DOB: [MM/DD/YYYY]
Claim number: [Claim #]   Date(s) of service: [MM/DD/YYYY]
Provider: [Name, NPI]

The claim above was denied as incurred before coverage. Coverage
was in effect on the date of service:
[ ] Coverage was made effective retroactively to [date]
    ([Medicaid approval / newborn enrollment / reinstatement]).
[ ] Your eligibility record shows the wrong start date. The
    correct effective date is [date].

Please update the eligibility record if needed and reprocess
the claim.

Enclosed: remittance advice, claim copy, [eligibility response
dated (date) / enrollment confirmation / approval notice].

[Name, credentials]  [Practice, NPI]  [Phone / email]

CO-26 prevention checklist

• Run eligibility for every new patient and every new card, for the exact date of service.
• Record the effective date, not just “active”, in the PM system.
• Keep the prior coverage on file until the new plan’s start date.
• For Medicare patients, check the Part B start date before the first visit.
• Track newborns until they’re added to a parent’s plan, then rebill.
• Flag pending Medicaid applications and hold or rebill once approved.
• Recheck the date of service on the claim before resubmitting.
• Watch the timely filing limit while you wait for retro coverage (see CO-29).

Related denial codes

• CO-27: coverage ended before the date of service.
• CO-31: the payer can’t identify the patient as a member.
• CO-109: the claim went to the wrong payer or contractor.
• CO-24: covered under a capitation or Medicare Advantage plan.
• CO-29: the timely filing limit has passed.

FAQ

Sources

• X12, Claim Adjustment Reason Codes (CARC 26, 27, 28, 31, 109). CARC 26 Start 01/01/1995. Accessed 7 Oct 2026.
• X12, Remittance Advice Remark Codes (N30, N52, N687). Accessed 7 Oct 2026.
• WPS Government Health Administrators, Common Claim Denials (last updated Dec 23, 2024)
• CMS, HIPAA Eligibility Transaction System (HETS)
• eCFR, 42 CFR 435.915, Medicaid effective date
• eCFR, 45 CFR 146.117, Special enrollment periods
• CMS, Medicare Claims Processing Manual (IOM 100-04), Ch. 22, §60.1 Group Codes

Last reviewed: 7 October 2026 · This page is educational and doesn’t replace payer-specific instructions.