CO-21 denial code
This injury is the liability of the no-fault carrier
How to fix it
Obtain the no-fault carrier, claim number and adjuster, then bill that carrier first.
How to prevent it
Ask whether an injury was motor-vehicle related at registration; no-fault benefits also exhaust, so bill promptly.
In practice
A patient is treated after a motor vehicle collision and the health plan returns CO-21, identifying auto no-fault or personal injury protection coverage as primary for the injury.
No-fault differs from ordinary liability in a way that matters operationally. Benefits are payable regardless of who caused the collision, so there is no fault determination to wait for, but they are capped — commonly at a fixed dollar amount that varies substantially by state.
Bill the no-fault carrier promptly with the claim number and adjuster details. Once the cap is exhausted, the health plan becomes responsible for the remainder, and the exhaustion letter from the auto carrier is what the health plan will require to accept it.
What sits behind it
The cap is the operational fact to plan around. Personal injury protection limits differ widely between states, and in a serious collision the benefit can be exhausted by the emergency department and initial imaging before an outpatient practice sees the patient at all.
Because benefits are consumed in the order claims arrive rather than by clinical priority, delay is expensive. A practice that bills the auto carrier six weeks late may find nothing left, while faster-billing providers have already drawn the fund down.
State rules add a second layer that generalisation cannot cover. Some states allow patients to elect health coverage as primary for auto injuries, some require the auto carrier to pay first without exception, and filing deadlines with auto carriers are frequently shorter than health plan deadlines. Verifying which regime applies is part of working the denial.
Related codes
Terms used here — Coordination of Benefits · Timely Filing · Denial
How we handle it — AR Management · Denial Management · Eligibility Verification
Primary sources
The rules behind CO-21, at the bodies that publish them.
- Workers' compensation medical billing (opens in a new tab)
US Department of Labor, OWCP — Federal workers' compensation fee schedule and billing requirements. State programs differ, but the structural rules are the same: a separate payer with its own schedule.
- 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 Claims Processing Manual (opens in a new tab)
Centers for Medicare & Medicaid Services — The operative manual for how Medicare claims must be coded, submitted, adjusted and appealed. When a payer policy and a vendor's advice disagree, this settles it.
Looking for a different code? Search all 190 CARC and RARC codes
Questions about CO-21
CO-20 points at a general liability carrier, where someone else's fault must usually be established before payment. CO-21 points at auto no-fault or personal injury protection, which pays regardless of fault but only up to a capped amount. The second is faster to collect and more likely to run out.
The health plan becomes responsible for the balance, but it will want proof. Request an exhaustion letter from the auto carrier stating the benefit is depleted and the date, then submit it with the claim to the health plan as documentation of the coordination sequence.
Immediately. Benefits are drawn down in the order claims are received, not by clinical importance, and auto carrier filing deadlines are often shorter than health plan deadlines. Every week of delay is another week for other providers to consume the available limit.
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