The Codes Behind America's Denials

Every denied or docked claim in the United States arrives speaking the same short language: a Claim Adjustment Reason Code, or CARC — CO-50, CO-197, CO-16— printed on the remittance advice next to the dollars that didn’t come. The list is maintained by X12, the standards body behind healthcare’s electronic transactions, and its own description of the codes is brief:

Which codes carry America’s denial volume? Here the honest answer comes first: nobody publishes that number. There is no public national dataset that counts denials at the CARC-code level — not from CMS, not from the payers, not from the standards body. What does exist is one regulated disclosure: ACA marketplace insurers must report their in-network denials by category to CMS, and KFF tabulates the result each year. It is the only published distribution of denial reasons in the country, and it covers one market segment — so it is the map this piece draws, labeled as exactly that.²

Each block below is sized by its share of reported reasons. Inside each territory sit the CARC families a provider typically sees there — each linked to its working page in our code library. Note what the proportions do to intuition: the categories practices argue about most, prior authorization and medical necessity, are the smallest named territories. The largest is a shrug.

The map rewards a slow read. “Other” — more than a third of everything reported — is not a code family; it is the absence of one, and it is why no CARC-level national count can be reconstructed from the disclosure data. The administrative quarter is where CO-16 (claim lacks information), CO-29 (filing deadline passed), and CO-18 (duplicate) live — denials about the claim’s paperwork rather than the patient’s care. And the smallest block on the map, medical necessity at 5%, is where the highest-stakes fights happen: CO-50 and CO-167 are the codes where a payer disputes clinical judgment itself.

The strip under the map matters as much as the map. CO-45, CO-97, and CO-4 are adjustment and bundling families: the claim is often marked paid, just for less, so these codes barely register in denial-category reporting — while consuming real hours at real desks. A map of reported denial reasons is not a map of provider pain.

The provider-side survey data tells the complementary story. In Experian Health’s 2025 State of Claims survey of 250 revenue-cycle professionals, the top reported causes of denials were missing or inaccurate claim data (50%, up from 46% in 2024), authorizations (35%), and incomplete or inaccurate patient registration data (32%).³ Translated into code families, that is CO-16 territory first and CO-197 territory second — the administrative and prior-auth blocks, seen from the receiving end. What providers report fighting and what issuers report denying are different measurements of the same machine, and neither one is a CARC-level count.

Whatever arrives on your remittance advice, it decomposes the same way: a group code that assigns financial responsibility, a reason code that states the payer’s why, and often a remark code adding detail. Read all three before deciding whether to appeal — the group code alone can change the answer.