CO · Contractual ObligationCARC 146

Denial Code CO-146

Q: What does CO-146 mean?

CO-146 is a Contractual Obligation (CO) denial: "Diagnosis was invalid for the date(s) of service reported." The ICD-10 diagnosis on the claim isn't valid for the date of service — typically because the code wasn't effective yet, was deleted or replaced in the annual update, or needs greater specificity for that date.

Q: Is CO-146 the patient's responsibility or a provider write-off?

CO-146 carries the CO group code (Contractual Obligation): Provider/contractual responsibility — not billable to the patient while denied. A CO adjustment is the provider's responsibility, not the patient's: balance-billing the patient for a CO amount is a contract (and often compliance) violation. But CO does not automatically mean “write it off.” When the cause is fixable — missing information (CO-16), a modifier problem (CO-4), or NCCI bundling (CO-97) — you correct and resubmit, or appeal with documentation. You only truly write the amount off when it is a final contractual adjustment, such as the fee-schedule difference on CO-45.

Diagnosis was invalid for the date(s) of service reported.

Source: X12 Claim Adjustment Reason Codes (CARC) & Group Codes. Maintained by the D3rx Clinical Billing Team.

What CO-146 means

The ICD-10 diagnosis on the claim isn't valid for the date of service — typically because the code wasn't effective yet, was deleted or replaced in the annual update, or needs greater specificity for that date.

Why CO-146 happens

An ICD-10 code that wasn't yet effective, or was deleted, for the date of service
A code from the wrong fiscal-year code set (the October 1 ICD-10 cutoff)
A placeholder or unspecified code used where a more specific one is now required
A truncated or otherwise invalid ICD-10 code

What to do when you get CO-146

1Verify the ICD-10 code is valid for the exact date of service using the correct fiscal-year code set
2Check whether the code was replaced or deleted in a recent annual update
3Select the correct, current, fully specified diagnosis supported by the documentation
4Resubmit a corrected claim with the valid diagnosis for that date of service

Got this denial right now?

Fix & resubmit: see the CO-146 correction steps

See CO-146 correction steps Ask D3 about CO-146

CO-146 clears with a corrected claim, not an appeal. Ask D3 walks you through the exact fix — free, no signup.

CO group code: who absorbs the charge

Contractual Obligation — Provider/contractual responsibility — not billable to the patient while denied.

A CO adjustment is the provider's responsibility, not the patient's: balance-billing the patient for a CO amount is a contract (and often compliance) violation. But CO does not automatically mean “write it off.” When the cause is fixable — missing information (CO-16), a modifier problem (CO-4), or NCCI bundling (CO-97) — you correct and resubmit, or appeal with documentation. You only truly write the amount off when it is a final contractual adjustment, such as the fee-schedule difference on CO-45.

Appeal, correct, or write off CO-146?

CO-146 is a coding correction, not an appeal: replace the diagnosis with the code that is valid and specific for the date of service and resubmit. Appeal only if the code billed was in fact valid for that date and the payer's edit misfired. Never substitute a diagnosis the record doesn't support to clear the edit.

Timing & deadlines

Resubmit the corrected claim within the timely-filing limit (Medicare 12 months from date of service; commercial ~90-180 days). Claims that span the October 1 code-set change are the most prone to CO-146, so check the date-of-service code year carefully.

Example

A claim with a December date of service is submitted using an ICD-10 code that was deleted in the October 1 update. The payer returns CO-146. Replacing it with the current replacement code and resubmitting resolves the denial.

Prevent CO-146 going forward

Update ICD-10 code sets every October 1 and retire deleted codes
Validate diagnosis effective/end dates against the date of service in your scrubber
Code to the highest specificity the documentation supports
Watch claims that straddle the fiscal-year boundary for code-validity issues

Code families most affected

Claims spanning the October 1 ICD-10 annual update
Diagnosis-driven coverage services (labs, imaging)
Any service where ICD-10 specificity governs payment

Related codes

Denial codes you'll often see alongside CO-146

CO-11 CO-16 CO-167

Payer notes

CO-146 is strictly about diagnosis validity for the date of service — distinct from CO-11 (diagnosis/procedure mismatch) and CO-167 (diagnosis not covered). The fix is a current, valid, specific ICD-10 for that exact date.

Fix this CO-146 denial the right way

CO-146 is resolved with a corrected claim, not an appeal. Ask D3 gives you the exact correction steps — free, backed by CMS, Medicare, and major-payer data.

See CO-146 correction steps Ask D3 about CO-146 All denial codes

Medical billing disclaimer

CARC/RARC definitions are the standardized X12 set; group-code semantics follow the X12 Claim Adjustment Group Code standard. Filing and appeal windows vary by payer — the standard anchors shown (Medicare redetermination 120 days from the remittance; Medicare timely filing 12 months from date of service; most commercial payers ~90-180 days) are general references, not a guarantee for any specific plan. Always confirm the rule in the payer's provider manual before acting. D3rx is not responsible for claim outcomes.