CO-146 Denial Code: Diagnosis Was Invalid for the Date(s) of Service
CO-146 means the diagnosis was invalid for the date(s) of service reported — the ICD-10 code was expired, not yet effective, truncated, or mistyped for that date. It carries the CO group code, so the amount is a contractual obligation the patient cannot be billed for; it is correctable by fixing the diagnosis and resubmitting.
- Group
- CO — Contractual Obligation
- Category
- Invalid / expired diagnosis code
- Appealable?
- Yes — after correcting to a valid ICD-10 for the DOS
- Typical fix
- Update to a valid, fully specified ICD-10; resubmit
What does denial code CO-146 mean?
CO-146 tells you the diagnosis reported was not valid on the date the service was rendered. The official X12 description is "Diagnosis was invalid for the date(s) of service reported." The ICD-10 code was expired, not yet effective, truncated to too few characters, or simply mistyped. Because it carries the CO group code, the denied amount is a contractual obligation — you cannot shift it to the patient.
It is a coding-validity error, not a benefit or medical necessity decision, which means it is fully correctable and recoverable.
Why does CO-146 happen?
- Expired code — the ICD-10 was deleted in an annual update effective October 1 and used past its end date.
- Not yet effective — a newly added code billed for a date before it took effect.
- Truncated code — an unspecified stem used where the payer requires full specificity (extra characters).
- Typo — a transposed or mistyped character producing an invalid code.
Mini-example: 99213 ($92) billed with ICD-10 S52.5 for a February date of service. S52.5 requires further characters and a 7th-character extension, so it is invalid as billed. The payer returns CO-146. Recoding to the fully specified S52.501A recovers the $92.
How do you fix a CO-146?
- Check the billed diagnosis against the ICD-10 code set effective on the date of service.
- Confirm the code is fully specified — correct laterality, encounter, and any required 7th character.
- Recode to the valid diagnosis supported by the documentation, and review the RARC remark codes for specifics.
- Resubmit as a corrected claim; if the original code was actually valid, appeal with the ICD-10 effective dates attached.
How do you prevent CO-146?
Load the current ICD-10 tables every October 1 and retire deleted codes from your favorites and templates. Turn on specificity edits in your scrubber so truncated stems are caught before submission, and cross-check unfamiliar denials with the denial code lookup. Because an invalid diagnosis often travels with an invalid procedure, review CO-146 lines alongside any CO-181 on the same claim.
Frequently asked questions
No. CO-146 carries the CO group code, so it is a contractual obligation, not patient responsibility. The claim denied because of a coding error on the provider side, not a coverage limit. Correct the diagnosis to a valid ICD-10 for the date of service and resubmit; the balance stays with the provider until then.
The ICD-10 code either did not exist yet, was retired before the date of service, or was billed at an incomplete level of specificity (truncated). ICD-10 updates take effect October 1 each year, so a code valid last year may be deleted or expanded this year. The code must be valid on the specific date the service occurred.
CO-146 is about the diagnosis code being structurally invalid for that date — expired, truncated, or nonexistent. A medical necessity denial (often CO-50) means the code is valid but does not justify the service billed. Fixing CO-146 is a coding correction; a necessity denial requires documentation supporting the service.
Sources & further reading
Reviewed by the ImmediCare Solutions RCM team
Certified billers and coders handling claims across 50+ specialties nationwide. This entry is reviewed against current payer policy and CMS rules. Last review: Jul 5, 2026.
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