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CO-146 Denial Code: Diagnosis Invalid for Date of Service

CO-146 means the diagnosis was invalid for the date of service reported. The ICD-10-CM code was not valid on that date: it may have been deleted, replaced, not yet effective, or not coded to the highest required level of specificity.

Quick facts

Code
CO-146 (CARC 146)
Status
Active In use since June 30, 2002; last modified September 30, 2007.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The provider must correct the diagnosis coding. The amount is not billable to the patient.
Official description
Diagnosis was invalid for the date(s) of service reported.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-146 means

CARC 146 says diagnosis was invalid for the date(s) of service reported. ICD-10-CM codes have effective dates. Codes are added, deleted, and expanded each year. The payer checked your diagnosis codes against the code set valid on the date of service, and at least one did not pass.

Timing is the usual culprit. A claim for a service before the October update must use the old code set; a claim for a service after it must use the new one. Mixing them triggers CO-146.

Example: a code is expanded in the October update to require an additional character. A practice keeps billing the shorter code for November services, and those claims return CO-146. The practice updates the diagnosis to the new, more specific code and resubmits.

Common causes

  • A deleted diagnosis code still in the provider’s favorites list or the EHR problem list.
  • A new code used for dates of service before it took effect.
  • A code missing required characters, such as a seventh character or placeholder X.
  • A category or header code billed instead of a complete, billable code.
  • A stale code set loaded in the billing or clearinghouse system.
  • A typo that turns a valid code into an invalid one.

How to fix it

  1. Identify the invalid code using the remark code and the claim line’s diagnosis pointer (box 24E).
  2. Look up the code in the ICD-10-CM code set for the date of service and find the valid code that matches the documentation.
  3. Update box 21 with the valid code and check that every pointer in box 24E still points to the right diagnosis.
  4. Resubmit. If the claim was rejected at the front end, send a new claim. If it was adjudicated, send a corrected claim with resubmission code 7 in box 22.
  5. If the code was valid on that date, contact the payer and request reprocessing, citing the code’s effective date.

How to prevent it

  • Load annual ICD-10-CM updates into your EHR, billing system, and clearinghouse before October 1.
  • Clean up provider favorites and problem lists after each update.
  • Hold claims that straddle the update so the correct code set is applied by date of service.
  • Run claims through a Claims Validator that checks code validity against the date of service.
  • For more on preventable coding denials, see medical claim denials and their preventable causes.

Remark codes that may appear with CO-146

  • M76 (Missing/incomplete/invalid diagnosis or condition.): The diagnosis or condition is missing, incomplete, or invalid.
  • M64 (Missing/incomplete/invalid other diagnosis.): A secondary (other) diagnosis is missing, incomplete, or invalid.
  • MA63 (Missing/incomplete/invalid principal diagnosis.): The principal diagnosis on an institutional claim is missing, incomplete, or invalid.
  • CO-181 (Procedure code was invalid on the date of service.): The procedure code, rather than the diagnosis, was invalid on the date of service.
  • CO-182 (Procedure modifier was invalid on the date of service.): The procedure modifier was invalid on the date of service.
  • CO-11 (The diagnosis is inconsistent with the procedure.): The diagnosis is valid but inconsistent with the procedure.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): General missing or invalid claim information.
  • CO-167 (This (these) diagnosis(es) is (are) not covered.): The diagnosis is valid but not covered.

CO-146 FAQ

When do ICD-10-CM codes change?

The main annual update takes effect October 1, and CMS can also make changes at other times of year. Claims must use the code set in effect on the date of service, not the date of billing.

Can a truncated diagnosis code cause CO-146?

Yes. A code that is missing required characters is not a valid billable code, even if its category exists. Code to the highest level of specificity available.

Do I need to appeal CO-146?

Usually not. Correct the diagnosis to a code valid on the date of service and send a corrected claim. Appeal only if the code was valid and the payer's file is wrong.