CO-28 denial code
Coverage not in effect at the time the service was provided
How to fix it
Confirm the coverage gap, identify any other active coverage, and rebill or transfer to patient responsibility.
How to prevent it
Re-verify before every visit. A verification performed at the previous encounter says nothing about this one.
In practice
A patient attends a follow-up in March. Verification was performed at their January visit and showed active coverage, so nobody checked again. The claim returns CO-28 — the policy terminated on 31 January.
Coverage terminations are invisible unless you look. Nothing arrives to announce them, the card in the patient's wallet is unchanged, and the patient frequently does not know either, particularly where the termination followed a job change or a missed premium.
Once confirmed, the account has two possible destinations: a different payer if replacement coverage exists, or patient responsibility if it does not. Finding replacement coverage is worth real effort, because retroactive Medicaid enrolment and marketplace special enrolment both routinely cover dates already past.
What sits behind it
The distinction between this and the closely related codes is worth holding. CO-28 means no coverage existed on the service date; CO-27 means coverage ended before it; CO-200 means the date fell inside a lapse, typically for unpaid premium; and CO-26 means it fell before the policy started. Each implies a different next step.
Grace periods are where the most recoverable money sits. Marketplace plans carry a mandated grace period for subsidised enrolees, and claims denied during it become payable if the premium is subsequently paid. Those denials are not final and should not be written off on first receipt.
The preventive answer is verification cadence rather than verification quality. A check performed at the previous encounter says nothing about this one, and practices that verify before every visit rather than at intake see this denial fall to a fraction of its former volume.
Related codes
Terms used here — Eligibility Verification · Coordination of Benefits · Patient Responsibility
How we handle it — Eligibility Verification · Denial Management · Patient Collections
Primary sources
The rules behind CO-28, at the bodies that publish them.
- Medicare Secondary Payer Manual (opens in a new tab)
Centers for Medicare & Medicaid Services — When Medicare pays second, and to whom the claim goes first. Coordination-of-benefits denials are resolved here rather than with the patient.
- Medicare Coverage Database (LCD/NCD) (opens in a new tab)
Centers for Medicare & Medicaid Services — Searchable national and local coverage determinations. The direct answer to whether a diagnosis supports medical necessity for a given procedure.
- Claim Adjustment Reason Codes (CARC) (opens in a new tab)
X12 — The authoritative, maintained CARC list. Our denial code lookup explains these in plain English; X12 is where the canonical definitions live.
Looking for a different code? Search all 190 CARC and RARC codes
Questions about CO-28
Often, yes. Retroactive Medicaid enrolment, marketplace special enrolment periods, and grace period reinstatements all make dates already past payable after the fact. Establishing whether any replacement coverage exists is worth doing before the balance is transferred to the patient.
CO-27 states that coverage terminated before the service, which identifies a specific end date. CO-28 states more generally that no coverage was in effect on that date, which can also mean it had not yet begun. The investigation is similar but the answer differs.
Before every visit, not at registration. Terminations happen silently between encounters and neither the card nor the patient reliably reflects them. Automated verification against the schedule makes per-visit checking practical without adding staff time.
Find out what your denials are costing you
A free billing audit reviews your denial rate, AR aging and clean claim rate against industry benchmarks. Takes about two minutes to request. No sales pitch.
No setup fees · You pay when we collect · Pricing from 3% of net collections