Two kinds of codes
HIPAA's medical code sets (ICD-10, CPT, HCPCS, CDT, and NDC) describe the patient's condition and the services delivered (45 CFR 162.1002). This guide covers the other kind: the code lists inside X12 transactions that describe the request and its outcome. These are the codes patient access and billing staff read every day.
Service type codes (eligibility)
A 270 eligibility inquiry can ask about a specific type of service using a service type code (STC), and the 271 reports benefits by STC. The federally adopted CORE rules require plans to support a defined list of 51 STCs (CORE 260). Common ones:
| STC | Name | Typical use |
|---|---|---|
| 30 | Health Benefit Plan Coverage | Generic check: is coverage active? |
| 1 | Medical Care | General medical benefits |
| 98 | Professional (Physician) Visit - Office | Office visit copay and coinsurance |
| 47 | Hospital | Hospital benefits in general |
| 48 | Hospital - Inpatient | Inpatient admissions |
| 50 | Hospital - Outpatient | Outpatient procedures and infusions |
| 86 | Emergency Services | Emergency department visits |
| UC | Urgent Care | Urgent care visits |
| MH | Mental Health | Behavioral health visits |
| 33 | Chiropractic | Chiropractic care |
| 35 | Dental Care | Dental benefits |
| AL | Vision (Optometry) | Vision benefits |
| 88 | Pharmacy | Pharmacy benefits |
A generic STC 30 inquiry must return benefits for a required set of service types, including 33, 47, 48, 50, 86, 98, and UC (CORE 154). Medicare's HETS answers unsupported STCs as if STC 30 had been sent (HETS Companion Guide).
AAA reject codes (eligibility)
When a payer cannot process a 270, the 271 contains an AAA segment with a reject reason code. The CORE rules require specific codes for specific failures so the provider knows what to fix (CORE 259).
| Code | Meaning | Usually means |
|---|---|---|
| 42 | Unable to Respond at Current Time | Payer system issue: retry later |
| 43 | Invalid provider identification | NPI not recognized or not enrolled with the payer |
| 51 | Provider Not On File | Provider not enrolled with this payer |
| 58 | Invalid/Missing Date of Birth | Date of birth mismatch |
| 62 | Date of Service Not Within Allowable Inquiry Period | Date too far in the past or future |
| 72 | Invalid/Missing Subscriber/Insured ID | Wrong or missing member ID or MBI |
| 73 | Invalid/Missing Subscriber/Insured Name | Name does not match the payer's records |
| 75 | Subscriber/Insured Not Found | No match: wrong payer or plan |
| 76 | Duplicate Subscriber/Insured ID Number | Multiple matches: add identifiers |
For Medicare, the HETS companion guide assigns each code a follow-up action, such as "retry later" for 42 and "correct and resubmit" for data errors, and notes that code 43 can mean the NPI does not have a valid Medicare enrollment (HETS Companion Guide; CMS).
Group codes, CARCs, and RARCs (payments)
On an 835 remittance, every adjustment pairs a group code, which says who is responsible for the difference, with a Claim Adjustment Reason Code (CARC), which says why. Remittance Advice Remark Codes (RARCs) add detail (Medicare Claims Processing Manual, Ch. 22).
| Group | Name | Meaning |
|---|---|---|
| CO | Contractual Obligation | A contract or regulation caused the adjustment; generally not billed to the patient. |
| PR | Patient Responsibility | An amount that may be billed to the patient or insured. |
| OA | Other Adjustment | Used when no other group code applies. |
| PI | Payor Initiated Reduction | A reduction the payer initiated. |
A fifth group code, CR (Corrections and Reversal), is not used with version 5010 and later (X12; X12 group codes).
The Claim Adjustment Status Code Maintenance Committee maintains CARCs, and CMS maintains RARCs. Both lists are updated on or around March 1, July 1, and November 1 (Medicare Claims Processing Manual, Ch. 22; X12 RARCs).
CARCs that start at the front desk
Many denials trace back to intake, eligibility, or authorization. These CARCs are among the ones patient access can prevent (X12 CARCs):
| CARC | Description (X12) | Front-end cause |
|---|---|---|
| 197 | Precertification/authorization/notification/pre-treatment absent. | No prior authorization on file |
| 27 | Expenses incurred after coverage terminated. | Coverage not re-verified before the visit |
| 109 | Claim/service not covered by this payer/contractor. You must send the claim/service to the correct payer/contractor. | Wrong payer, such as Medicare Advantage billed as traditional Medicare |
| 204 | This service/equipment/drug is not covered under the patient's current benefit plan | Benefit not checked for the service type |
| 242 | Services not provided by network/primary care providers. | Network status or referral not checked |
| 16 | Claim/service lacks information or has submission/billing error(s). | Missing or wrong registration data |
| 50 | These are non-covered services because this is not deemed a 'medical necessity' by the payer. | Documentation does not support the service |
CARC 15, "The authorization number is missing, invalid, or does not apply to the billed services or provider," was deactivated on May 1, 2018, so older references to it may be out of date (X12 CARCs).
CARCs that reflect cost share
Some CARCs are not denials at all. They report the patient's share that the 271 predicted (X12 CARCs):
- 1: Deductible Amount
- 2: Coinsurance Amount
- 3: Co-payment Amount
- 45: Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement. X12 says to use it only with group code PR or CO.
Comparing these amounts with the cost share recorded at eligibility is a quick check on whether the front-end estimate was right.
Acknowledgment and decision codes
Before any business response, a sender may receive a TA1 (envelope errors), a 999 (syntax errors within a functional group), or, for claims, a 277CA (claim-level acceptance or rejection). These are not HIPAA-mandated but are widely used (Medicare Claims Processing Manual, Ch. 24).
A 278 prior authorization response reports its decision with action codes such as A1 (certified in total), A3 (not certified), A4 (pended), and A6 (modified) (CMS esMD 278 Companion Guide).
Using codes to prevent denials
- Track AAA rejections by code. Repeated 72s or 73s point to registration problems; repeated 75s often mean the wrong payer.
- Check eligibility with the specific STC for the service, not only STC 30.
- Review CARC 197, 27, 109, and 242 denials with the intake team, since each has a front-end cause.
- Keep code references current: CARC and RARC lists change three times a year.
Saga's agents verify eligibility, flag problems such as an inactive plan or out-of-network provider for staff review, and track prior authorizations to a decision, which addresses several of these denial causes before the claim is sent. See eligibility verification and prior authorization.
Sources
- eCFR: 45 CFR Part 162, Administrative Requirements (HIPAA transaction standards)
- CAQH CORE 154: Eligibility and Benefits 270/271 Data Content Rule (Phase I)
- CAQH CORE 259: Eligibility and Benefits 270/271 AAA Error Code Reporting Rule (Phase II)
- CAQH CORE 260: Eligibility and Benefits Data Content (270/271) Rule (Phase II)
- CMS: HETS 270/271 Companion Guide (version 15-1, May 2026)
- CMS: About HETS 270/271
- X12: Claim Adjustment Reason Codes
- X12: Remittance Advice Remark Codes
- X12: Claim Adjustment Group Codes
- CMS: Medicare Claims Processing Manual, Chapter 22 (Remittance Advice)
- CMS: Medicare Claims Processing Manual, Chapter 24 (General EDI and EDI Support Requirements)
- CMS: esMD X12N 278 Companion Guide (version 19.0, January 2026)
This guide is for general information and is not legal or compliance advice. Check the cited CMS sources and your payers’ current policies for requirements that apply to you.