What to Do When Your Claim Is Denied with CO-50 (Medical Necessity Denied)

How to Understand the Code, Fix the Issue, and Appeal Successfully

If you received an Explanation of Benefits (EOB) with CO-50, you’re not alone. CO-50 is one of the most common denial codes in U.S. health insurance—and also one of the most frequently overturned when a proper appeal is filed.

But here’s the catch:

Most CO-50 denials get overturned only when the patient (or provider) submits a complete, well-supported medical necessity appeal.

In this article, I’ll walk you through:

• What a CO-50 denial actually means

• Why insurers use it so often

• How to fix the problem

• How to gather the medical evidence insurers expect

• How to write a strong appeal letter

• What deadlines apply under state law and ERISA

• When to escalate the issue

• The easiest way to appeal (using my attorney-drafted template)

Let’s break it down clearly and step-by-step.

1. What Does CO-50 Mean?

CO-50 = The payer has denied the claim because the service was “not medically necessary.”

In plain English:
The insurer is saying the care was not needed, not justified, or not supported by documentation.

Common services denied under CO-50:

• Physical therapy sessions

• Imaging (MRI, CT, ultrasound)

• Lab tests

• Out-of-network treatment

• Mental health therapy

• Durable medical equipment

• Emergency room visits

• Specialist consults

• Hospital admissions

Importantly:

A CO-50 denial does NOT mean the insurer is right—it only means they didn’t see enough medical justification.

2. Why Insurance Companies Deny Claims With CO-50

Insurers use CO-50 aggressively because it gives them a financial incentive to flag services as unnecessary.

Here are the most common reasons insurers cite:

Reason 1: Missing or incomplete medical records

Most CO-50 denials happen because the insurer never received:

• Chart notes

• Test results

• Referral notes

• Physician rationale

• Diagnosis codes

Reason 2: Wrong or vague diagnosis codes

If the diagnosis doesn’t match the treatment (for example: back pain + MRI of the knee), insurers deny it.

Reason 3: Not meeting policy criteria

Example:

• Therapy requires preauthorization

• Imaging requires “step therapy” (X-ray first)

• A medication requires failure of alternatives

Reason 4: Services billed longer than insurer guidelines

Many insurers have proprietary “medical necessity guidelines” that doctors don’t always know.

Reason 5: Delay or incorrect coding

Sometimes the patient did everything right but the office coded it poorly.

Good news:

Most CO-50 denials are reversible with a proper appeal.

3. Your First Step: Read the Denial Letter Carefully

Your EOB only gives the code.

The denial letter gives:

• The insurer’s detailed reasoning

• The policy section they relied on

• Your deadline to appeal

• The address for submission

• Whether internal or external review applies

Look for these phrases:

• “Our records do not support medical necessity…”

• “Documentation not received…”

• “Does not meet criteria under policy section…”

• “Alternative treatments required first…”

You need to know exactly why the insurer denied the claim before appealing.

4. What You Need to Gather Before Submitting an Appeal

A successful CO-50 appeal includes three types of evidence:

A. Medical Records (required)

Request:

• Office visit notes

• Test results

• Imaging reports

• Treatment plan

• Follow-up notes

• Provider referral

You have the right to these under HIPAA.

B. A Letter of Medical Necessity From Your Doctor

This is the #1 most important document in reversing a CO-50 denial.

It should explain:

• Your diagnosis

• Your symptoms

• What treatments were tried

• Why this treatment was required, not optional

• What would happen if you didn’t receive it

I can generate a template for this if you want.

C. The Insurance Policy Section

Attach:

• The policy’s definition of “medical necessity”

• The criteria for the specific service

Then show how your case meets the policy requirements.

5. How to Write a Strong CO-50 Appeal (Step-by-Step Script)

Here is the exact structure you should follow:

1. Opening Paragraph

State that you are appealing a medical necessity denial, include:

• Patient name

• Member ID

• Claim number

• Date of service

• Denial code (CO-50)

2. Summary of Why the Denial Is Incorrect

Example:

“The treatment was medically necessary, supported by clinical findings, and meets your policy’s criteria for the service.”

3. Provide a Timeline of Symptoms and Treatment

Detail:

• Onset

• Previous failed treatments

• Provider recommendations

• Why alternative treatments were not appropriate

4. Cite the Policy Section

Quote the insurer’s definition of medical necessity.
Then show how you meet every element.

5. Attach Supporting Evidence

State:

• Records enclosed

• Physician letter attached

• Imaging reports

• Test results

6. Demand a Reconsideration

Close with:

“Please conduct a full, fair, and thorough review of this appeal under all applicable federal and state laws, including ERISA if this is a group plan.”

6. Your Deadlines: Know Them or You Lose Your Rights

ERISA Plans (most employer plans):

• 60 - 180 days to appeal

• Must receive a written decision within 30–60 days

• Right to request your complete claim file

• Right to external review in many cases

Non-ERISA plans (individual policies / Medicare / Medicaid):

Deadlines vary by state but are often:

• 60–120 days to appeal

• External review available

Critical:
If you miss your deadline, you may lose your right to contest the denial.

7. When to Request an External Review

If your internal appeal is denied, request an Independent Review Organization (IRO) review.

These are legally binding in many states.
IROs frequently overturn CO-50 denials because they rely on medical evidence, not insurer cost-saving policies.

8. The Fastest Way to Appeal a CO-50 Denial

Instead of drafting everything from scratch, you can use the attorney-drafted Medical Necessity Appeal Template I created specifically for CO-50 and medical necessity denials.

This includes:

• A full professionally written appeal

• Instructions for attaching medical evidence

• Citations to ERISA and state consumer protection laws

• Fields you fill in (diagnosis, treatment, dates, provider info)

If you'd like, I can:
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✔ Create your CTA section

9. Denied for “Medical Necessity”? Fix It Before Your Deadline Expires.

A CO-50 denial won’t go away on its own — and missing key language can cost you your claim. My Medical Necessity Appeal Letter Template gives you everything you need to submit a strong, timely appeal.

✔ Attorney-written
✔ Proven to work
✔ Instant access

👉 Get your Medical Necessity Appeal Template here.

You CAN Win Your Appeal

A denial letter is not the final decision. Insurance companies overturn claims every day when people submit a strong appeal.

Whether you use:

➡ The Free Appeal Guide
or
➡ The $29 Professional Appeal Template

—you can absolutely take back control and fight your denied claim with confidence.

Download Your Free Appeal Guide

Get the step-by-step instructions, evidence checklist, and sample wording you need.
Download Free Guide

Need a Full Appeal Letter Template? ($29)

Get a professionally drafted, fill-in-the-blank appeal letter tailored for medical claim denials.
Get the Full Template