CO-4 — Modifier missing or inconsistent with the procedure
Claim-adjustment reason code 4 · typically reported as CO-4
What it means
The HCPCS code needs a modifier it didn’t get, or the modifier billed doesn’t make sense for that code.
Why DME claims hit it
- Missing RR/NU/UE rental-vs-purchase modifier on a DME item
- Missing KX when the medical-policy criteria attestation is required
- Missing LT/RT on paired orthotics or prosthetics
- Capped-rental month modifiers (KH/KI/KJ) out of sequence
How to fix it
- Look up the required modifier set for the code (payment category drives RR/NU/UE)
- Correct the modifier and resubmit as a corrected claim — this is usually a clean fix
Appeal posture
Resubmit corrected rather than appeal; appeals are for disputes, not data errors.
Related denial codes
CO-182 — Modifier invalid on the date of serviceCO-16 — Claim lacks information or has a submission error
Looking up an item instead? Browse the HCPCS code library for fee schedule amounts and PA flags.
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