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ABA claim denials, code by code

Each guide starts from the official X12 code definition, then walks through why ABA claims land there and what to check before you resubmit.

Sourced guides for ABA billers and owners, organized by the problem you are solving.

5 guidesUpdated October 2026Sourced from primary documents

Tip. Start with the guide that matches the denial or audit question in front of you.

A denial on an 835 remittance has three parts: a group code (who is responsible, for example CO for Contractual Obligation), a Claim Adjustment Reason Code (CARC, the why), and usually one or more Remittance Advice Remark Codes (RARC, the detail). X12 maintains those code lists. The guides below quote the definitions and translate them into checks an ABA billing team can run. Guides for specific CPT code situations live here too.

Guides in this section

CO-197: authorization absent on an ABA claimCO-197 means precertification or authorization was absent. Why ABA claims get it, how it differs from CARC 198, 284, 296 and 302, and a pre-claim checklist.CO-16: claim lacks information or has a billing errorCO-16 means the claim lacks information or has a billing error. How to read the remark code that comes with it, the ABA causes, and how to prevent them.CO-50: not deemed a medical necessityCO-50 means the payer says the service is not medically necessary. What ABA documentation supports medical necessity, and how Florida defines it.Billing 97153 and 97155 on the same day, or at the same timeCan you bill 97153 and 97155 at the same time? What CPT guidance and the ABA Coding Coalition say, how Florida Medicaid handles it with modifier XP, and how other payers differ.The HN modifier on 97153 and other ABA codesWhat the HN, HM and HO HCPCS modifiers mean, how commercial payers apply them to 97153, how Florida Medicaid uses HN, and the denials a wrong modifier causes.
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Common questions

What does CO-197 mean on an ABA claim?

X12 defines CARC 197 as precertification/authorization/notification/pre-treatment absent. With group code CO (Contractual Obligation), the payer found no matching authorization and the amount cannot be billed to the family.

What does denial code CO-16 mean?

X12 defines CARC 16 as claim/service lacks information or has submission/billing error(s). The payer must send at least one remark code that explains what is missing or wrong.

What does CO-50 mean?

X12 CARC 50: these are non-covered services because this is not deemed a medical necessity by the payer. Group code CO means the amount is a contractual adjustment.

Can 97153 and 97155 be billed at the same time?

Per the ABA Coding Coalition, yes, when both code descriptors are met and two different practitioners deliver them; a single QHP may not report both concurrently. Payers differ: Kentucky and Oregon Medicaid allow it with conditions, while Florida Medicaid's FFS fee schedule reports the supervisee's overlapping time with modifier XP, not reimbursed.

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