835 Denial Combination
CO-4+N170
Contractual Obligation · Service-Line Level Adjustment
Coding ErrorWhat This Combination Means
The claim was denied because the procedure code and modifier combination submitted are inconsistent with each other, and the payer requires a certificate of medical necessity (CMN) to support the service. The N170 remark indicates that even if the coding error is corrected, documentation proving medical necessity must be submitted. This typically occurs with durable medical equipment (DME) or supplies where both proper coding and supporting CMN documentation are required for payment.
Financial Responsibility
provider writeoff
The provider must write off the denied amount per contractual obligation. The patient cannot be billed for this adjustment.
N/A
Appeal Success
Corrected claim (30-60 days)
Avg. Resolution
Medium
Difficulty
No
Appealable
Step-by-Step Resolution
Steps tailored specifically to this CO-4+N170 combination — not generic advice.
- 1
Identify the specific modifier-procedure code mismatch
Review the Healthcare Policy Identification Segment (loop 2110 Service Payment Information REF) in the 835 ERA if present to understand which modifier is incompatible with the procedure code billed
- 2
Obtain or renew the certificate of medical necessity
Secure a new, revised, or renewed CMN from the ordering physician that supports the medical necessity of the service and aligns with the correct procedure code
- 3
Correct the procedure code or modifier combination
Adjust the claim to reflect the appropriate modifier that is consistent with the procedure code per payer coding guidelines
- 4
Submit a corrected claim with the CMN attached
File the corrected claim with both the accurate coding and the required certificate of medical necessity documentation to support payment
Specialty Context
How CO-4+N170 typically presents across different practice types.
Dental
Medical
Common with durable medical equipment claims where a procedure code for equipment or supplies is billed with an incorrect modifier (e.g., rental vs. purchase modifiers) and requires a physician-signed CMN to demonstrate ongoing need
Behavioral Health
Individual Code References
View the standalone definition for each code in this combination.
Medicare Contractor Guidance for CARC 4
FCSO + Noridian + uhc + aetna + bcbs_azThis CARC code is received when a claim is submitted and the procedure code(s) are billed with the wrong modifier(s), or the required modifier(s) are missing. A clear understanding of Medicare's rules and regulations is necessary to assign the appropriate modifier(s) correctly.
How to Prevent CARC 4 Denials
- ✓
Before submitting your claim, ensure you use the most current year's CPT codes and modifiers.
- ✓
Know the proper use of the CPT modifiers that exist and the appropriate use for the specific condition or situation.
- ✓
Check the claim to verify if an applicable modifier(s) is warranted and has been applied to the procedure code billed.
- ✓
Providers can use the Modifier lookup tool which provides information for most procedure code modifiers used by Medicare.
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If a modifier has been entered but the Medicare contractor rejects the claim, verify that the correct modifier(s) was/were used.
- ✓
Submit separate claims for services in different years of service. A procedure code or modifier valid in one year may not be valid in the other and will cause the entire claim to reject or deny.
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