What are CAS adjustment reason codes in an EDI 835 file?

CAS adjustment reason codes appear in CAS segments of an EDI 835 file and explain why a portion of a claim or service line was not paid as billed. Each CAS entry includes a group code such as CO, PR, OA, or PI and a numeric reason code that describes the specific type of adjustment, like a deductible, contractual write-off, or non-covered service.

How are CAS segments structured in the EDI 835?

A CAS segment starts with an adjustment group code followed by one or more triplets of reason code, adjustment amount, and optional quantity. For example, CAS*CO*45*10.00*1 tells you there is a CO-45 adjustment for $10.00 with a quantity of 1. Multiple triplets can be repeated in the same CAS segment to represent several adjustments.

What is the difference between CO, PR, OA, and PI group codes?

CO indicates contractual obligations such as write-offs dictated by fee schedules or network agreements. PR represents patient responsibility such as deductibles, copays, and coinsurance. OA stands for other adjustments that do not fit neatly into the other categories, and PI typically reflects payer-initiated reductions, policy decisions, or internal processing rules.

How do CAS adjustment reason codes relate to CARC codes?

The numeric reason codes used in CAS segments are often referred to as Claim Adjustment Reason Codes (CARCs). These codes are maintained by industry code lists and payer companion guides. When you see a CAS group code and a CARC value together, they map to a standardized description that explains the adjustment in more detail.

How can I use CAS adjustment codes in denial and underpayment analysis?

By grouping claims and service lines by CAS group and reason codes, you can identify common denial reasons, recurring contractual reductions, and patient responsibility patterns. Many organizations use CAS codes to track denial volumes, trend underpayments, and prioritize follow-up or root-cause analysis in revenue cycle workflows.

Why do I sometimes see multiple CAS segments for the same claim or service line?

A single claim or service line may have several distinct reasons for not paying the full billed amount. Payers can express these using multiple CAS segments or multiple triplets within one CAS. When analyzing the 835, you typically need to sum all related CAS amounts to fully reconcile the difference between the billed, allowed, and paid amounts.

CAS Adjustment Reason Codes: Complete Guide for EDI 835

Quick Table of Contents

What are CAS adjustment codes?
CAS groups explained (PR, CO, OA, PI)
Most common CAS reason codes
Full CAS adjustment reason code table
Using CSV/Excel to analyze adjustments
FAQ: CAS adjustments

What are CAS adjustment reason codes?

CAS (Claim Adjustment Segment) reason codes explain why a payer reduced, adjusted, or denied part of a claim or service line. You’ll see them in EDI 835 files attached to CLP (claim-level) or SVC (line-level) segments.

Example CAS segments

CAS*PR*1*20.00*1~
CAS*CO*45*35.00*1~

These show two adjustments:

PR‑1 Deductible: $20.00
CO‑45 Contractual write‑off: $35.00

CAS codes are essential for billing, appeals, underpayment analysis, and patient responsibility breakdowns.

CAS groups (PR, CO, OA, PI)

CAS codes are divided into four adjustment groups:

Group	Name	Meaning
PR	Patient Responsibility	Deductibles, coinsurance, copays. Amounts the patient owes.
CO	Contractual Obligations	Write-offs required by payer contracts or fee schedules. Not billable to patient.
OA	Other Adjustments	Miscellaneous payer adjustments not part of patient responsibility or contract terms.
PI	Payer Initiated	Payer-driven reductions, often administrative or policy related.

Quick rule: PR = patient owes, CO = provider adjusts, OA/PI = payer-specific rules.

Most common CAS adjustment reason codes

Code	Group	Description
PR‑1	Patient Responsibility	Deductible
PR‑2	Patient Responsibility	Coinsurance
PR‑3	Patient Responsibility	Copay
CO‑45	Contractual	Charge exceeds fee schedule/maximum allowable
CO‑97	Contractual	Benefit included in allowance for another service
OA‑23	Other	Payment adjusted due to impact of prior payer(s)
PI‑204	Payer Initiated	Service not covered under payer policy

These account for the majority of adjustments in typical commercial and Medicare/Medicaid ERAs.

Full CAS adjustment reason code list

This is a copy-friendly, billing-friendly version of the full official reason code set.

Note: The official reason code set is maintained by CMS. This table reflects the current operational set used across payers.

*This table includes the most widely used codes. Full CMS master lists include additional rarely-used values.

Code	Description
1	Deductible Amount
2	Coinsurance Amount
3	Copayment Amount
4	The procedure code is inconsistent with the modifier used
5	The procedure code is inconsistent with the place of service
6	The procedure/revenue code is inconsistent with the patient’s age
7	The procedure/revenue code is inconsistent with the patient’s gender
8	The procedure/revenue code is inconsistent with the provider type
16	Claim lacks information required for adjudication
18	Duplicate claim/service
45	Charge exceeds fee schedule/maximum allowable
49	These are non-covered services
50	These services are not medically necessary
53	Service/procedure not furnished directly to the patient
96	Non-covered charge(s)
97	Benefit included in payment/allowance for another service
151	Payment adjusted because the payer deems the information submitted does not support this level of service
204	Service not covered under payer policy
252	Interest payment
253	Sequestration reduction (2% Medicare reduction)

Using CSV/Excel to analyze adjustments

CAS codes become far more useful when converted into columns. A flat table lets you analyze:

Top denial reasons
Total patient responsibility (PR group)
Total contractual write‑offs (CO group)
Underpayments by CPT, provider, facility
Trends over time by payer

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FAQ: CAS Adjustment Codes

Where do CAS codes appear in an 835?

They appear after CLP segments (claim-level adjustments) and after SVC segments (line-level adjustments).

Are CAS codes the same as denial codes?

Not exactly—CAS codes explain adjustments. Some indicate denials, others explain write‑offs, policy limitations, or benefit rules.

Which CAS codes are billable to the patient?

Only PR (Patient Responsibility) codes are billable. CO, OA, and PI adjustments are not billable.

Which CAS codes matter most for denial analytics?

Common denial codes include CO‑45, CO‑97, PI‑204, OA‑23, and reason codes like 16 (missing information).

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