CO-146 denial code
Diagnosis was invalid for the dates of service reported
How to fix it
Recode using the code set in effect on the date of service and resubmit.
How to prevent it
ICD-10-CM updates each October 1. Claims for services before that date must use the prior code set.
In practice
A practice bills a September encounter in mid-October using a diagnosis code introduced in that year's update. The claim returns CO-146: the code did not exist on the date of service.
ICD-10-CM changes take effect on 1 October each year, and validity is judged against the encounter date rather than the submission date. A code effective this October cannot describe a service delivered in September.
Recoding with the classification in force on the service date resolves it. The recurring version of this problem is a system holding only the current code table, which makes correct billing for prior dates structurally impossible.
What sits behind it
The October boundary catches practices twice. Claims for late September services submitted in October need the previous code set, and claims held for any reason across the boundary need it too, which means the exposure runs for as long as the filing window stays open.
Code changes are not limited to additions. Codes are deleted, split into more specific variants and revised in their inclusion terms, so a code that remains in the classification may still be invalid for a date if its definition changed at a boundary.
The distinction from CO-47 is worth noting because the remedies differ. CO-146 is specifically about date validity, while CO-47 also covers diagnoses that are valid but not covered for the procedure. Reading which arrived tells you whether the problem is a table or a policy.
Related codes
Terms used here — ICD-10-CM · Claim Scrubbing · Clean Claim
How we handle it — Medical Coding · Claims Management · Practice Analytics
Primary sources
The rules behind CO-146, at the bodies that publish them.
- CPT code set (opens in a new tab)
American Medical Association — Maintainer of CPT. Annual changes to CPT are the most common cause of a sudden, unexplained rise in denials each January.
- Claim Adjustment Reason Codes (CARC) (opens in a new tab)
X12 — The authoritative, maintained CARC list. Our denial code lookup explains these in plain English; X12 is where the canonical definitions live.
- National Correct Coding Initiative (NCCI) edits (opens in a new tab)
Centers for Medicare & Medicaid Services — The procedure-to-procedure and medically-unlikely edits behind most bundling denials. Checking these before submission prevents the denial entirely.
Looking for a different code? Search all 190 CARC and RARC codes
Questions about CO-146
On 1 October each year, with the new codes valid from that date forward and the previous set governing everything before it. Claims for September services submitted in October must use the earlier classification, which is where most instances of this denial originate.
For as long as the filing window remains open for prior-year dates, which for many payers means several months and for some considerably longer. Systems that overwrite the code table at update make correct historical billing impossible and will produce this denial every year.
CO-146 is specifically about validity on the date of service. CO-47 is broader and also covers diagnoses that are valid but not covered for the procedure billed. The first is a code table problem; the second may be a coverage policy question.
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