CO-252 means the payer wants a document before it pays, such as notes, an operative report, an invoice or a certificate of medical necessity. The remark code names what is missing. Send it the way the payer asks, often through its portal or with the claim's attachment control number.
The official wording
The X12 Claim Adjustment Reason Code list defines 252 as: "An attachment/other documentation is required to adjudicate this claim/service. At least one Remark Code must be provided."
The letters in front are the group code. CO means contractual obligation (the provider absorbs it), PR means patient responsibility, and OA is other adjustment. The same reason code can come with different group codes, so always read both.
Common causes
- Unlisted procedure codes that always need notes.
- Modifier 22 or other modifiers that need proof of extra work.
- Drugs or supplies that need an invoice.
- High-dollar or unusual claims that trigger review.
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How to fix it
- Read the remark code. It names the document.
- Send exactly that document, with the claim number on every page.
- Use the payer's method: portal upload, fax cover sheet, or electronic attachment with the PWK segment.
- Follow up in two weeks to confirm it was matched to the claim.
What a CO-252 looks like
A surgeon bills an unlisted procedure code. The payer cannot price it without the operative report, so it holds payment.
| Line on the remittance | Amount | What it means |
|---|---|---|
| Billed charge | $2,400.00 | Unlisted procedure |
| CO-252 with N350 | $2,400.00 | Payer needs a description of the service before it can pay |
| Paid by payer | $0.00 | Nothing yet |
Once the report reaches the payer and is matched to the claim, the line is priced and paid. No appeal is needed.
Remark codes that tell you which document
X12 requires at least one remark code with reason 252. That remark code is your instruction.
| Remark code | Official wording | Send this |
|---|---|---|
| M23 | "Missing invoice." | The supplier or manufacturer invoice for the drug, device or supply |
| M127 | "Missing patient medical record for this service." | The visit or procedure note for that date |
| N350 | "Missing/incomplete/invalid description of service for a Not Otherwise Classified (NOC) code or an Unlisted procedure." | The operative report or a clear description of the service, with the drug name and dose for NOC drugs |
| N29 | "Missing documentation/orders/notes/summary/report/chart." | The order, note or report the payer named in its letter or portal |
| N706 | "Missing documentation." | Check the payer letter or portal. It will say which record |
How to send attachments electronically
Most payers match paperwork to a claim with an attachment control number. On the 837P it goes in the PWK segment in loop 2300 (claim level) or 2400 (line level).
| PWK part | What goes there | Example |
|---|---|---|
| Report type | What the document is | OZ (support data), 09 (progress report), OB (operative note) |
| Transmission | How you will send it | FX (fax), BM (mail), EL (electronic) |
| Control number | Your own unique ID | The same number you write on the fax cover sheet |
Send the claim with the PWK, then send the document within the payer's window, often 5 to 10 days. Put the control number and claim number on every page.
CO-252 checklist
- ☐ You read the remark code and know which document is needed.
- ☐ The document is for the same patient, date and provider as the claim.
- ☐ The claim number or attachment control number is on every page.
- ☐ You sent it the way the payer asks (portal, fax or PWK).
- ☐ You set a 14-day follow-up to confirm the payer matched it.
Questions people ask about CO-252
What is the difference between CO-252 and CO-16?
CO-252 is for missing attachments or documents. CO-16 is for missing or wrong data on the claim itself.
Can I bill the patient for CO-252?
No. Send the documentation to the payer.
Why does CO-252 always come with a remark code?
X12 requires at least one remark code with reason 252, because the remark code names the missing document.
What is the PWK segment?
It is the part of the electronic claim that tells the payer paperwork is coming. It holds the report type, how you will send it and your attachment control number.
Do I need to appeal a CO-252?
Usually no. Send the requested document and the payer reprocesses the claim. Appeal only if you sent it and the payer still denied.
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All denial code guides
PR-1 · PR-2 · PR-3 · CO-4 · CO-5 · CO-11 · CO-15 · CO-16 · OA-18 · CO-22 · OA-23 · CO-24 · CO-26 · CO-27 · CO-29 · CO-31 · CO-45 · CO-49 · CO-50 · CO-59 · PR-96 · CO-97 · CO-109 · CO-119 · CO-129 · CO-140 · CO-146 · CO-151 · CO-167 · CO-170 · CO-181 · CO-185 · CO-197 · CO-198 · PR-204 · CO-234 · CO-236 · CO-242 · CO-252 · CO-253 · CO-288 · CO-A1 · CO-B7 · MA130 · N130 · N23 · N290
Timely filing limits by payer: Cigna timely filing limit · HealthSpring timely filing limit · Aetna timely filing limit · UnitedHealthcare timely filing limit · Humana timely filing limit · corrected claim timely filing limits
Sources: X12 Claim Adjustment Reason Code list (252), read October 27, 2026; CMS Medicare guidance where Medicare rules are mentioned; Noridian Medicare JE Part B reason code guidance ("Missing Invoice", CO-252 with M23), read October 27, 2026; X12 RARC list (M23, M127, N350, N29, N706); X12 837P implementation guide (PWK segment). Last reviewed October 27, 2026. General information, not coding or legal advice. Payer rules differ, so check the remark code and the payer's own policy.