CO-252 Denial Code: Attachment Required, How to Fix It

CO-252 is a Claim Adjustment Reason Code that means the payer needs an attachment or other documentation before it can decide the claim, and the CO group code keeps the amount off the patient’s bill. (Primary sources: X12 Claim Adjustment Reason Codes · CMS Medicare Claims Processing Manual, Ch. 12 · Noridian JE Part B)

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Quick answer: CO-252 means the payer couldn’t adjudicate the line without a document it didn’t get, such as an invoice, an operative note or a test identifier. The remark code names the missing item. Send it the way the payer asks, or correct and resubmit when the remark says so. Don’t bill the patient.

Part of our denial codes library. Written by Manikandan, a revenue cycle management specialist with more than 10 years of US medical billing experience. Last reviewed: 6 October 2026. Official code wording is quoted word for word from X12; Medicare rules link to their CMS or Medicare contractor source.

CO-252 denial code description (official X12 wording)

X12 Claim Adjustment Reason Code 252, verbatim:

“An attachment/other documentation is required to adjudicate this claim/service. At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT).”

Source: X12 Claim Adjustment Reason Codes, checked 6 October 2026. Start: 09/30/2012 | Last Modified: 06/02/2013.

252 is X12’s code for missing documentation. The usage note on CARC 16 says, “Do not use this code for claims attachment(s)/other documentation.” So when paperwork is the issue, payers are supposed to send 252, not CO-16.

What CO-252 means in plain English

CO (Contractual Obligation) means the provider is responsible for the amount. CMS describes CO adjustments as “generally… considered a write off for the provider and are not billed to the patient” (Ch. 22, §60.1).
CARC 252 means the payer stopped because a document was missing. It hasn’t decided the service isn’t covered or isn’t necessary.
The remark code is required: X12 requires at least one non-Alert RARC with 252. It names the missing piece, such as M23 (invoice) or N706 (documentation).

A close relative is CARC 251: “The attachment/other documentation that was received was incomplete or deficient. The necessary information is still needed to process the claim.” 252 means nothing usable arrived. 251 means something arrived but it wasn’t enough.

CO-252 vs PR-252 vs OA-252 vs PI-252

Group + 252	Who carries the amount	Can you bill the patient?	Notes
CO-252	Provider	No	The usual form. The provider controls the documentation, so the provider carries the delay
PR-252	Patient	Only for the amount shown, under your contract	Uncommon. Read the remark code to see what’s needed and from whom
OA-252	Neither group fits	Check the remark code first	Used “when no other group code applies” (CMS Ch. 22, §60.1)
PI-252	Payer-initiated	No	Non-Medicare remits only. Medicare remittances use CO, OA and PR

Remark codes that come with CO-252

RARC	Official X12 text	What it points to	Published by
M23 (+ N704)	“Missing invoice.”	The invoice price wasn’t reported, or was in the wrong format, on an invoice-priced code	Noridian JE Part B, Missing Invoice
N706 (+ N704)	“Missing documentation.”	For molecular tests: a missing or invalid MolDX DEX Z-Code identifier	Noridian JE Part B, DEX Z-Code
N706	“Missing documentation.”	Modifier 22 billed without supporting documentation	CMS Claims Processing Manual, Ch. 12, §40.4
N704	“Alert: You may not appeal this decision but can resubmit this claim/service with corrected information if warranted.”	Tells you to resubmit, not appeal	Paired by Noridian with M23 and N706
M127	“Missing patient medical record for this service.”	The payer wants the chart note for the date of service	X12 RARC list (general use)

RARC wording is from the X12 RARC list, checked 6 October 2026. The CMS manual entry for modifier 22 reads: “Group Code: CO, CARC: 252, RARC: N706.” M127 is listed for reference. We didn’t find a Medicare contractor table pairing it with 252.

Where CO-252 shows up in the 835

A trimmed, illustrative 835 service line for an invoice-priced item billed at $640.00 (XXXXX stands for the HCPCS code):

SVC*HC:XXXXX*640.00*0.00**1~
DTM*472*20260908~
CAS*CO*252*640.00~
LQ*HE*M23~
LQ*HE*N704~

CAS*CO*252 carries the adjustment.
LQ*HE*M23 names the missing item: the invoice.
LQ*HE*N704 tells you this line can’t be appealed but can be resubmitted.

If you sent documents with the original claim, also check whether your 837 carried a PWK segment and whether the payer logged the attachment. A payer can’t link paperwork it can’t match to a claim.

How to fix CO-252: decision path

Remark codes M23, N706 and N704 with PWK timeframes, and what to send or resubmit

Read every remark code. M23, N706 and M127 ask for different things. N704 tells you the route: resubmit, not appeal.
Decide whether it’s a claim data fix or a document send. Noridian’s M23 fix is a data fix: report the “total invoice price plus shipping cost (do not include handling or other fees)” in Item 19 of the CMS-1500 or its electronic equivalent. Its N706 MolDX fix is also data: report the DEX Z-Code in loop 2400, SV101-7.
For a document send, use the payer’s channel. For Medicare that’s the PWK process, an ADR response or esMD. For commercial plans, it’s usually an EDI 275 attachment or a portal upload.
Match the identifiers. Put the claim number, or the attachment control number from your PWK segment, on every page. Noridian says the ACN “must exactly match the ACN entered on the PWK Coversheet.”
Resubmit when N704 or MA130 is present. Medicare treats these lines as not appealable, so a new claim with the fix is the way forward. Resubmitting doesn’t stop the 12-month Medicare filing clock (42 CFR 424.44).
Appeal only when the payer had the document. If your PWK or 275 record shows the payer received complete documentation on time, request a reconsideration with that proof attached.

Medicare and major payer notes

Medicare PWK. The PWK segment in loop 2300 (claim) or 2400 (line) tells the MAC that paperwork is coming. Noridian lists the required parts: PWK01 report type (for example OB for an operative note), PWK02 transmission method (BM mail, FX fax, EL or FT esMD), PWK05 “AC,” and PWK06, your attachment control number. The claim waits only for a set time: 10 calendar days for mail and 7 for fax or esMD. Noridian says these timeframes “are designated by CMS and cannot be altered,” and late documents “will not be used for the claim processing” (Noridian JE Part B, PWK). PWK is optional and doesn’t force a review. Noridian also says not to send a PWK coversheet for invoice-priced services, because it “does not require the physical copy of the invoice.”

Medicare ADRs. An Additional Documentation Request is a different process from a 252 on the remit. National Government Services (its site now carries the Wellpoint Federal name) reminds providers to respond to ADRs within “45 days from the date of request” (NGS top claim errors). Our MAC prepayment review guide covers ADR responses.

Modifier 22. CMS tells MACs to use CO-252 with N706 “when a physician submits a claim with modifier ‘-22’ but does not provide additional documentation” (Ch. 12, §40.4). Send the operative report with every modifier 22 claim.

UnitedHealthcare. UHC accepts unsolicited EDI 275 claim attachments through listed clearinghouses, including Availity and Change Healthcare, and says providers “can continue to upload supporting documentation (solicited and unsolicited) for all claims through the UnitedHealthcare Provider Portal” (UHC, May 2025).

Other plans. Attachment channels and deadlines differ by payer. Check the provider manual for the payer’s 275, portal or fax process and the time limit for unsolicited attachments before you send.

Worked example (illustrative)

A practice bills Medicare Part B for an item the MAC prices by invoice. The biller attaches the invoice to the claim in the PM system but leaves Item 19 blank. The remit shows:

CAS*CO*252*640.00 with LQ*HE*M23 and LQ*HE*N704

Reading it: the MAC needed the invoice price on the claim, not the paper. N704 means no appeal.

Fix: submit a new claim with the price in Item 19, or its electronic equivalent, in the format Noridian shows, for example “Invoice $640.00.” The format matters. Noridian’s own examples show “Invoice $130” pricing at $1.30, while “Invoice $130.00” prices at $130.00.

Prevention: add a claim edit that blocks invoice-priced codes when Item 19 is empty.

CO-252 documentation cover letter template

Use this when the payer asks for records or you’re sending an attachment after a 252. Replace the bracketed text.

[Date]
To: [Payer name], [Attachments / Claims Documentation unit]
Via: [EDI 275 / portal upload / fax number / esMD]

Re: Documentation for claim [claim # / ICN], denied CO-252 [RARC]
Patient: [Name]   Member ID: [ID]   DOB: [MM/DD/YYYY]
Date of service: [MM/DD/YYYY]   Line(s): [CPT/HCPCS + modifiers, billed $]
Attachment control number (if PWK used): [ACN]

The remittance dated [date] adjusted the line(s) above with CARC 252 and
remark code [RARC], requesting [item named by the RARC]. Enclosed:

1. [Document, e.g. operative report dated MM/DD/YYYY] - [n] pages
2. [Invoice / test identifier / order] - [n] pages

Each page shows the patient name, date of service and claim number.
Please reprocess the line(s) using the enclosed documentation.

[Name, title]  [Practice, NPI, Tax ID]  [Phone / email]

For Medicare lines that carry N704 or MA130, send a corrected new claim instead of a letter.

CO-252 prevention checklist

Keep a list of codes your payers price by invoice or review by documentation, and require the data before the claim releases.
Put invoice prices in Item 19, or its electronic equivalent, in the exact format your MAC shows.
Attach the operative report to every modifier 22 claim.
For MolDX tests, store the DEX Z-Code with each test in your charge master.
When you use PWK, send the documents right after you get the claim number, within 7 days by fax or esMD and 10 by mail.
Track solicited requests (ADRs, payer letters) with due dates in one work queue.
Separate CO-252 from CO-16 in denial reports so documentation gaps don’t hide among data errors.

CO-16: claim lacks information or has a billing error. X12 sends attachment problems to 252 instead.
MA130: unprocessable claim, no appeal rights. Submit a new claim.
CO-251: the documentation received was incomplete or deficient.
CO-50: the payer reviewed the records and found the service not medically necessary.
CO-96: non-covered charge. A coverage decision, not a paperwork one.
Unlisted codes: see what to put in Box 19 for unlisted procedures.

FAQ

What is the CO 252 denial code description?

X12 describes CARC 252 as “An attachment/other documentation is required to adjudicate this claim/service.” It also requires at least one remark code that isn’t an Alert, which tells you what documentation is missing.

What does denial code CO 252 mean?

It means the payer needs a document before it can decide the claim, and the provider carries the amount until it’s resolved. It isn’t a coverage or medical necessity decision. The remark code, such as M23 or N706, names the missing item.

How do I fix a CO 252 denial?

Read the remark code. If it points to claim data, like an invoice price or a test identifier, correct the claim and resubmit. If it asks for records, send them through the payer’s attachment channel with the claim number on every page. Resubmit instead of appealing when N704 or MA130 appears.

Is CO-252 the same as CO-16?

No. CO-16 is for missing or invalid claim data. X12’s CARC 16 usage note says not to use 16 for “claims attachment(s)/other documentation,” so documentation problems belong under 252.

Can I appeal a CO-252 denial from Medicare?

Not when the line carries N704 or MA130. Those remarks mean you resubmit with the correction instead. If the payer had your complete documentation on time and still denied, a reopening or redetermination with proof of delivery is reasonable.

How long do I have to send PWK documents to Medicare?

Noridian’s PWK page says the claim is held for 10 calendar days when you mail the documents and 7 when you fax them or use esMD. Send them as soon as you have the claim number. Documents that arrive after the hold aren’t used for that claim.

Sources

Last reviewed: 6 October 2026 · This page is educational and doesn’t replace payer-specific instructions.