X12 835 Reference

X12 835 CAS01 (Claim Adjustment Group Code): Format, Values, and Testing Notes

Jul 28, 2026 · 4 min read

CAS01 is a single two-letter code, but it decides which bucket an adjustment dollar lands in — write it off, bill the patient, or chase another payer — and getting it wrong in test data means your billing statements and your contractual write-off reports are both being validated against the wrong scenario.

Quick answer

CAS01 is the first element in each CAS segment iteration of an X12 835. It identifies which party is responsible for the adjustment amount that follows, using one of four group codes, and every downstream posting decision — write it off, bill it to the patient, or pursue another payer — starts from this single value.

The four group code values

Contractual Obligation — the provider absorbs the difference under its contract with the payer — this is a write-off, not a patient bill

Patient Responsibility — the amount shifts to the patient — copay, coinsurance, deductible, or a non-covered service

Other Adjustment — an adjustment that does not cleanly fit contractual or patient categories, such as an interest payment or a prior overpayment recovery

Payer Initiated Reductions — the payer reduced the amount on its own initiative rather than because of a signed contract term

Example

Synthetic example Generated by Synthibase

CAS*PR*1*25.00

A synthetic patient-responsibility adjustment: CAS01 PR shifts $25.00 to the patient, reason code 1 identifies it as a deductible amount. Synthetic remittance data.

Where this trips people up

Synthetic test files built early in a project tend to lean almost entirely on CO, because contractual write-offs are the most common and the easiest to reason about. That leaves PR untested until a real patient statement comes out wrong, and OA and PI barely tested at all, even though a mapping engine that treats every group code as a write-off will happily zero out an amount that should have gone to the patient balance instead. Synthetic data needs all four group codes represented, not just the common one.

→

X12 835 CAS Segment Guide: Fields, Format, and Testing Gotchas

→

EDI 835 Remittance Advice: Complete Guide for Healthcare IT

→

How to Test 835 Remittance Reconciliation Before Go-Live

Test every adjustment group code, not just contractual write-offs

Synthibase generates synthetic 835 files spanning CO, PR, OA, and PI adjustments in realistic proportions, so your posting logic routes every dollar to the right bucket before go-live.

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This page is written independently by Synthibase for testing and informational purposes. It is not an official publication of HL7 International or X12/Washington Publishing Company, and is not a substitute for reviewing the official standard or your trading partner's companion guide. See our Terms of Service for full legal terms.