CO-28 arrives when a payer's records show no active coverage for the patient on the date of service. Most billing staff treat it as final, write the balance off or forward it to the patient, and move on. That's a mistake a surprising share of the time. A meaningful portion of CO-28 denials reflect a payer data error or a verification gap at the practice, not an actual coverage gap. Knowing which you have changes everything about how you respond.
What CO-28 means
- CO is the Claim Adjustment Group Code for Contractual Obligation. The write-off is governed by your payer contract, and you generally cannot bill the patient for it under the contract terms.
- 28 is the Claim Adjustment Reason Code: "Coverage not in effect at the time the service was provided."
The code describes what the payer found in its system. It does not tell you whether that finding is accurate. Payers carry enrollment data from employer groups, exchange plans, and government programs, and that data can be wrong or delayed. CO-28 means the payer's records say no coverage, not necessarily that no coverage existed.
The three causes behind most CO-28 denials
Getting to the right response requires knowing which of these you are dealing with.
- Payer data error. The patient has valid coverage but the payer's system shows a wrong termination date or hasn't processed a new enrollment. This is common after plan-year changes (January and open-enrollment periods), COBRA activations, or mid-year employer group switches. The patient is covered; the payer just doesn't know it yet.
- Stale insurance information at the practice. The patient brought an old card from a prior employer or a plan they dropped. The practice billed the wrong payer. The patient does have coverage, but not with the plan that got the claim.
- True coverage gap. The patient genuinely had no coverage on the DOS because they left an employer, missed a premium payment, aged off a parent's plan, or let COBRA lapse. This is the case where the CO-28 denial is substantively correct.
How to respond to a CO-28 denial
The first move is always verification, not a write-off. Work through this sequence before deciding anything:
- Run a real-time eligibility check for the exact DOS. Use your clearinghouse or the payer portal directly. Compare the coverage effective and termination dates in the response to the DOS. If the response shows active coverage on the DOS, you have a payer data error.
- Contact the patient for documentation. Ask for an enrollment confirmation letter, an HR benefits summary showing effective dates, or a new insurance card for the DOS. Patients whose CO-28 denial is actually a payer data error are almost always surprised and cooperative.
- If you confirm coverage existed, open a dispute. Call the payer's provider relations line, present the eligibility documentation, and request a reprocessing. Attach the supporting documents and resubmit as a corrected claim, citing the verification transaction ID if you have one from your clearinghouse.
- Track the appeal deadline. Even an eligibility dispute has a filing window. Confirm the payer's timely-appeal period and submit within it. Missing the window forfeits the claim regardless of merit.
Can you bill the patient after CO-28?
The CO group code technically prohibits passing the balance to the patient under most payer contracts. But the real answer depends on the situation. A CO-28 because the payer's data is wrong (the patient was covered) is different from a CO-28 because the patient genuinely had no insurance on the DOS.
If the coverage gap was real and the patient was uninsured, your financial agreement with the patient, signed at intake, typically allows you to collect for services rendered. Review that agreement and your state's patient billing rules before forwarding a balance. Do not automatically send a balance to the patient on a CO-28 before confirming which situation you have. Forwarding a balance on a payer data error, where the patient was actually covered, puts you at risk under the payer contract and damages the patient relationship.
Preventing CO-28 at the source
Real-time eligibility verification on the day of service resolves most of this before the claim is ever filed. A few practices that move the needle:
- Verify on the DOS, not just at scheduling. Coverage can lapse between the day a patient books and the day they arrive. A verification at scheduling tells you what was true then, not what is true at the visit.
- Save the eligibility transaction ID. Your clearinghouse returns a transaction reference number with every eligibility response. Storing that ID against the visit gives you documented proof of coverage at the time of service, which is the evidence you need to dispute a CO-28 payer data error.
- Flag plan-year transitions. January and open-enrollment months generate a spike in CO-28 denials because employers submit roster updates and payers process them on different schedules. In those months, reverify any patient whose employer-based plan changed.
- Check for a secondary payer. When the primary produces CO-28 and coverage really did lapse, the patient may have a secondary plan (spouse's employer plan, Marketplace coverage) that was the active plan on the DOS. Ask before assuming the patient is self-pay.
A note for Texas practices
When a CO-28 denial turns out to be a payer data error and you dispute it successfully, the corrected payment may be subject to Texas prompt-pay interest provisions if the payer is a state-regulated commercial plan and the corrected payment comes in late. Texas Insurance Code Chapter 1301 covers PPO plans; Chapter 843 covers HMOs. Track the date your clean corrected claim was accepted and calculate any interest accrual from there. Most practices never claim it; most payers never volunteer it.
CO-28 vs. CO-15
Both are administrative denials but for different reasons. CO-15 means an authorization number is missing, invalid, or doesn't apply to the service billed. CO-28 is an eligibility problem: the payer says the patient had no coverage. CO-15 is usually fixable with an authorization correction; CO-28 requires an eligibility investigation first.
What to do with a CO-28
Run the eligibility check before you do anything else. A surprising number of CO-28 denials resolve without an appeal simply because the payer's data was wrong and a resubmission with verification documentation gets the claim paid. The ones that don't are genuine coverage gaps, and knowing that clearly sets up the right conversation with the patient. For the broader picture of how eligibility and other administrative denials feed into revenue loss, the revenue-leakage guide covers the full pattern. See also how silent denials hide in paid claims for the category of losses most billing workflows never see at all.