If you’ve been in medical billing long enough, you’ve probably had that frustrating moment — a claim gets denied, you check the EOB, and there it is: CO-252 staring back at you.
This one doesn’t say “not covered” or “invalid code.” It says something even more annoying:
“An attachment or other documentation is required to adjudicate this claim.”
In other words, the payer isn’t rejecting the service—they’re just saying, “We don’t have enough proof to pay you.”
The CO-252 denial isn’t about coding errors or eligibility issues. It’s about missing documentation—the kind of stuff that should have come with the claim but didn’t. And while it’s one of the easiest denials to fix, it’s also one of the most common revenue blockers for healthcare practices.
A single CO-252 can delay reimbursement for weeks, especially if no one catches it early. Multiply that across multiple claims, and you’re looking at serious cash flow issues.
In this guide, we’ll break down what CO-252 really means, why it happens, and how to fix it fast — so you can get those claims paid and keep your revenue cycle moving smoothly.
What is the CO-252 Denial Code in Medical Billing?
When you receive an Explanation of Benefits (EOB) or Electronic Remittance Advice (ERA) from a payer and see the code CO-252, it’s their way of saying:
“We can’t process this claim because we need more documentation.”
The full description of CO-252 is:
“An attachment or other documentation is required to adjudicate this claim.”
In simpler terms, the payer didn’t get enough paperwork to verify the service, so they stopped the claim in its tracks until you sent it. It’s like trying to get a driver’s license without bringing your ID — no matter how complete the form is, the application can’t move forward until you provide what’s missing.
The CO part of the code stands for Contractual Obligation. This means the provider, not the patient, is responsible for fixing the issue. The payer isn’t denying the claim because of medical necessity or eligibility — they’re just waiting for proper documentation.
So, when you see CO-252:
- You cannot bill the patient for it.
- You must send the missing documentation and resubmit the claim.
- Once the payer receives what they need, they’ll reprocess the claim for payment.
What Documentation is Required?
Every insurance payer needs proof that the service you billed for was legitimate, medically necessary, and properly performed. Documentation serves as that proof.
For example:
- A surgery claim might need the operative note.
- A therapy session might need progress notes.
- A DME claim (like a wheelchair or oxygen supply) might need a doctor’s prescription or Certificate of Medical Necessity (CMN).
- A lab test might need an order from the referring physician.
If any of those pieces are missing, the payer can’t confirm the details and that’s when CO-252 appears.
Why Payers Use This Denial Code
Payers use CO-252 as a “pause button.” It’s not a permanent denial; it’s more like a request for missing evidence.
They’re basically saying:
“Hey, we got your claim, but before we can cut the check, show us the paperwork that proves this was justified.”
They want to see:
- The medical reasoning behind the service.
- Documentation that matches the date, CPT/HCPCS code, and provider.
- Authorization or referral proof (if required).
- Any attachments required by policy — like progress notes or lab results.
Until they get that, the claim sits in limbo — unpaid, unprocessed, and unresolved.
Common Causes of CO-252 Denial
CO-252 denials usually stem from missing, incomplete, or mismatched documentation. Let’s go through the top culprits.
Claim Submitted Without Required Attachments
Some procedures, especially in specialties like radiology, pathology, therapy, or DME (Durable Medical Equipment), require documentation such as:
- Medical records or physician notes
- Operative reports
- Therapy progress notes
- Signed orders or prescriptions
- Certificates of medical necessity (CMNs)
- Diagnostic test results
- Referral or authorization documentation
If these aren’t attached (or properly linked electronically), the payer won’t process the claim.
Electronic Submission Error
Even if you did attach documents, sometimes the EDI (Electronic Data Interchange) system fails to transmit the attachments correctly — or the payer’s system doesn’t recognize them.
That’s especially common with clearinghouses or payers that require specific attachment control numbers or reference fields.
Claim Requires a PWK Segment
Some payers require a PWK (Paperwork) segment in electronic claims to indicate an attachment is being sent. If it’s missing or incorrect, the claim looks “unsupported,” triggering CO-252.
Documentation Sent Separately (and Not Matched)
If you mailed or faxed the paperwork after submitting the claim electronically, but didn’t include:
- The correct claim number, or
- The attachment control number (ACN)
The payer can’t match the documents to your claim.
Preauthorization or Referral Not Included
Some services need prior authorization or a specialist referral. If it’s missing or not linked, the payer requests it—and if you don’t send it, you’ll see CO-252.
Incomplete Operative or Procedure Notes
For surgical or diagnostic services, missing signatures, incomplete documentation, or mismatched CPT/ICD details often lead to this denial.
How to Fix CO-252 Denial Code in Medical Billing
When you see CO-252 on your EOB or ERA, don’t treat it like a lost cause. It’s not saying “we won’t pay you”—it’s saying “we need more info before we can pay you.”
The good news? You can fix it. The key is to figure out what’s missing, gather it fast, and resubmit the claim correctly.
Here’s precisely how to do that — step by step.
Review the EOB or ERA Details
Before doing anything, read the payer’s message carefully.
The EOB (Explanation of Benefits) or ERA (Electronic Remittance Advice) usually includes an additional remark code explaining what documentation they need.
Examples:
- N102: “Documentation was not received.”
- M51: “Missing or incomplete information.”
- N706: “Documentation required to support medical necessity.”
These remarks tell you precisely what the payer expects — it could be operative notes, therapy logs, medical records, or authorization documents.
Tip: Don’t assume. Different payers request different attachments for the same service. Always go by what the EOB says explicitly.
Identify What’s Missing
Next, determine which document is missing.
Common culprits include:
- Physician’s order or referral letter
- Preauthorization number
- Operative report or clinical note
- Lab or diagnostic results
- Certificate of Medical Necessity (CMN) for DME claims
- Progress or therapy notes for rehab services
- Proof of medical necessity (e.g., patient history or symptoms)
Check your EHR or paper file to see if you already have that documentation. Sometimes it was submitted but not transmitted correctly — that’s a transmission issue, not a missing document.
Check How You Submitted the Claim
If you sent the claim electronically, verify whether:
- You included a PWK (Paperwork) segment in your EDI file (this tells the payer you’re sending documentation).
- The Attachment Control Number (ACN) in the PWK segment matches the number on your attachment or fax cover sheet.
- Your clearinghouse successfully transmitted the attachment (check your clearinghouse acknowledgment report).
If the claim was submitted on paper, ensure the attachment is included and labeled correctly with the claim number, patient name, and date of service.
Pro tip: Some payers (like Medicare and BCBS) use automated scanners to match attachments. If you forget the control number or claim ID on your documents, the system can’t connect them — resulting in CO-252.
Gather and Prepare the Missing Documentation
Once you know what’s missing, gather it and prepare it for submission.
Here’s what you should include with every document:
- Patient name
- Date of service
- Provider name/NPI
- Claim control number or original claim ID
- Description of what’s attached (e.g., “Operative report for CPT 47562”)
If you’re faxing or mailing, include a simple cover sheet like this:
- Provider Name: Sunrise Medical Group
- NPI: 1987654321
- Patient: Maria Lopez
- Claim #: 76432981
- Date of Service: 09/15/2025
- Reason: CO-252 Denial – Missing Preauthorization Letter
- Attachments: Preauthorization Approval (Aetna Ref #A12345)
This helps payers quickly match your documentation.
Resubmit the Claim Correctly
Now it’s time to send the corrected claim.
- For CMS-1500 claims (professional) – use Frequency Code “7” in Box 22 (to indicate a corrected claim).
- For UB-04 claims (facility) – use Type of Bill (TOB) ending in “7” for a replacement claim (e.g., 117 for inpatient).
Attach the documentation either:
- Through the payer’s provider portal (fastest),
- Via the clearinghouse attachment upload,
- Or by fax/mail (include your cover sheet and claim number).
Do not submit it as a brand-new claim — otherwise, it will be denied again as a duplicate.
Follow Up With the Payer
Don’t assume it’s fixed just because you sent the documents.
Follow up after 10–15 business days to confirm:
- The payer received the documentation,
- The claim is reprocessing, and
- There are no further issues.
Ask for a reference number or call log ID for your follow-up. That record helps if you need to escalate the issue later.
Some payers (like UnitedHealthcare, Humana, and BCBS) offer online tools to check attachment status — use them!
Document Everything in Your Billing Notes
Always record the entire interaction in your billing system:
- Date CO-252 was received
- What documentation was missing
- When and how you resubmitted it
- Confirmation or reference number from the payer
This way, if the denial repeats or you need to appeal, you’ve got a clear paper trail.
Conclusion: CO-252 Isn’t a Denial — It’s a Delay You Can Control
When you strip away the jargon, CO-252 is basically a “missing paperwork” alert. It’s the payer’s way of saying: “We’re not saying no — we’re saying not yet.”
The fix is simple: identify what documentation is missing, attach it correctly, resubmit the claim as a correction, and follow up.
No appeals, no complicated coding corrections — just clean documentation management and good workflow habits.
The real power lies in prevention. Once your team starts double-checking documentation before claims go out — and learns how to use PWK segments or EDI attachment tools properly — you’ll see your CO-252 denials drop sharply.
In short:
- Understand what the payer wants.
- Send it right the first time.
- Keep a clean claim trail.
That’s how you turn CO-252 from a recurring headache into a rare inconvenience.
Let Medix Revenue Group Handle Co-252 Denial Code for You
Tired of chasing denials and paperwork trails? Medix Revenue Group specializes in helping healthcare practices eliminate documentation-related denials, such as CO-252, for good.
Our billing experts handle everything — from claim submission and attachment management to payer follow-ups and appeals. We don’t just fix denials; we prevent them by tightening your documentation workflow and integrating smarter billing automation.
Partner with us today to simplify your claim process, reduce denials, and get paid faster — without drowning in paperwork.
