CO-26 means the payer's records show the patient's coverage started after the date of service. Find the coverage that was active that day, or confirm with the patient that their effective date is wrong in the payer's system.
The official wording
The X12 Claim Adjustment Reason Code list defines 26 as: "Expenses incurred prior to coverage."
The letters in front are the group code. CO means contractual obligation (the provider absorbs it), PR means patient responsibility, and OA is other adjustment. The same reason code can come with different group codes, so always read both.
What CO-26 is telling you
The plan was not in force on the date of service, according to the payer. Look at the group code: with PR-26 the payer is saying the patient is responsible; with CO-26 check your contract before billing the patient.
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Common causes
- A new job or new plan where the visit happened during a waiting period.
- A retroactive enrollment that the payer has not loaded yet, common with Medicaid and marketplace plans.
- The wrong member ID on the claim, belonging to the new plan instead of the old one.
How to fix it
- Run eligibility for the date of service, not today's date.
- Ask the patient what coverage they had that day. Bill that plan.
- If the effective date is wrong, have the patient or employer correct it with the plan, then ask for reprocessing.
One claim before coverage started
A patient starts a new job on March 1. The new plan has a waiting period, so coverage starts April 1. The patient comes in on March 20 and gives the new card.
| Date | Event | Result |
|---|---|---|
| March 1 | New job starts | |
| March 20 | Visit, billed to the new plan | |
| April 1 | New plan coverage starts | |
| April 15 | Remittance | CO-26, $0 paid |
| Fix | Bill the coverage active on March 20, such as the old plan or COBRA |
Cases where the date is wrong, not the coverage
- Newborns. Plans must cover a newborn from birth when the parent adds the baby inside the plan's enrollment window. Until the baby is added, claims may deny with CO-26.
- Medicaid. Approval can be backdated to cover recent months. Rebill once the effective date shows in eligibility.
- Marketplace plans. Coverage starts only after the first premium is paid. Claims can deny until the payment posts.
How to fix CO-26
- Run eligibility for the date of service.
- Ask the patient which plan covered them that day.
- Bill that plan. Note its filing limit.
- If the effective date is wrong, ask the patient or employer to correct it, then request reprocessing.
- If there was no coverage, follow your self-pay policy.
CO-26 checklist
- ☐ Eligibility was checked for the date of service.
- ☐ You asked about the old plan, COBRA or Medicaid.
- ☐ Newborn claims wait until the baby is added to the plan.
- ☐ The patient is billed only if no coverage existed.
Questions people ask about CO-26
Can I bill the patient for CO-26?
Check the group code and your contract. If the payer sent PR-26, the patient is responsible. If it says CO-26, look at the contract first, and try to find the coverage that was active.
What is the difference between CO-26 and CO-27?
CO-26 means the service was before coverage started. CO-27 means it was after coverage ended.
Why did a newborn claim deny with CO-26?
Usually the baby has not been added to the parent's plan yet. Once the parent adds the baby, coverage goes back to the birth date and the claim can be resubmitted.
Can Medicaid coverage start before the visit even if it was approved later?
Yes. Medicaid approval can cover earlier months. Rebill once the eligibility shows the backdated start date.
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All denial code guides
PR-1 · PR-2 · PR-3 · CO-4 · CO-5 · CO-11 · CO-15 · CO-16 · OA-18 · CO-22 · OA-23 · CO-24 · CO-26 · CO-27 · CO-29 · CO-31 · CO-45 · CO-49 · CO-50 · CO-59 · PR-96 · CO-97 · CO-109 · CO-119 · CO-129 · CO-140 · CO-146 · CO-151 · CO-167 · CO-170 · CO-181 · CO-185 · CO-197 · CO-198 · PR-204 · CO-234 · CO-236 · CO-242 · CO-252 · CO-253 · CO-288 · CO-A1 · CO-B7 · MA130 · N130 · N23 · N290
Timely filing limits by payer: Cigna timely filing limit · HealthSpring timely filing limit · Aetna timely filing limit · UnitedHealthcare timely filing limit · Humana timely filing limit · corrected claim timely filing limits
Sources: X12 Claim Adjustment Reason Code list (26), read October 27, 2026; CMS Medicare guidance where Medicare rules are mentioned; Examples are for illustration; check each plan's enrollment rules. Last reviewed October 27, 2026. General information, not coding or legal advice. Payer rules differ, so check the remark code and the payer's own policy.