Denial Code 121 (CO-121, PR-121) — Meaning & What to Do
Claim Adjustment Reason Code (CARC)
What denial code 121 means: An outstanding balance from a previous period was applied.
What to do about denial code 121: Ask for an itemized history of the previous balance.
CO-121 vs PR-121 vs OA-121: what the prefix means
| Code on your EOB | What the prefix means | Who typically pays |
|---|---|---|
| CO-121 | Contractual obligation — the provider writes the amount off | Usually not the patient |
| PR-121 | Patient responsibility | The patient |
| OA-121 | Other adjustment | Often another payer or plan |
The prefix tells you who is expected to absorb the amount; the number (121) tells you why.
Seeing denial code 121 on your EOB?
A denial code tells you what the payer reported, but it may not tell the whole story. Reviewing your EOB and bill together can help clarify what may need attention.
Related codes
CARC 118Charges were reduced for a specific health program requirement.
CARC 122This is a non-covered charge because it's a routine/screening exam.
CARC 117Transportation is only covered to the nearest appropriate facility.
CARC 125The claim was submitted with an error in how it was set up.
Keep reviewing your bill
- Medical Bill vs EOB: What Is the Difference?
- How to Dispute a Medical Bill — Find Billing Errors Before You Pay
- What Is an Itemized Medical Bill? How to Request One
Frequently asked questions about Denial code 121
What does denial code 121 mean on my EOB?
An outstanding balance from a previous period was applied.
What should I do about denial code 121?
Ask for an itemized history of the previous balance.
Seeing this code on your own EOB?
Upload the EOB, the bill, or both — MD Bill Check reads the codes on your documents, compares them side by side, and flags what's worth questioning. Free, no account needed.