"Not medically necessary": how to appeal a CO-50 denial

Few words on a denial letter feel more final than "not medically necessary." It sounds like a verdict. It is not. A CO-50 denial is the insurer's clinical opinion that your service did not meet its coverage criteria, and an opinion can be challenged. A successful not medically necessary appeal does exactly that: it puts your treating physician's judgment, the published clinical evidence, and the insurer's own written policy in front of a reviewer who has to engage with all three. This guide shows you how to build that case and how to prove medical necessity in a way the insurer cannot brush aside.

What "not medically necessary" and CO-50 actually mean

On an Explanation of Benefits or remittance advice, the reason for the denial is encoded as a Claim Adjustment Reason Code, or CARC. CARC 50 reads, verbatim: "These are non-covered services because this is not deemed a medical necessity by the payer." The "CO" prefix means the contractual obligation falls on the provider, but the substance is the same: the payer has decided your service does not meet its definition of medical necessity.

The key word is "deemed." A medical necessity denial appeal starts from a simple truth: this is a judgment call made by the payer, usually by a reviewer applying internal criteria, sometimes without ever speaking to your doctor or reading your full record. Your plan documents contain a specific definition of medical necessity, often phrased as "consistent with generally accepted standards of medical practice," "clinically appropriate," and "not primarily for the convenience of the patient or provider." The denial is the insurer's claim that you do not meet that definition. Your appeal is the rebuttal.

Why a CO-50 denial is one of the most appealable

Medical-necessity denials are among the most appealable denials you can receive, for a structural reason: they turn on clinical judgment, and clinical judgment can be re-decided. A coding error or an eligibility problem is a yes-or-no fact. Medical necessity is an argument, and arguments can be won with better evidence.

Two features of the appeals system favor you here. First, because the denial involves "medical judgment," it generally qualifies for external review, where an independent physician, not employed by your insurer, gets the final say. Second, the law entitles you to see the exact criteria the insurer used, which means you can attack the denial on its own terms rather than guessing. Overturn rates vary widely by plan, state, and condition, so a single national figure would mislead more than inform. What is true is qualitative and important: when the clinical record genuinely supports the treatment, a well-built appeal is hard for a reviewer to uphold.

It also helps to understand why these denials happen in the first place, because the reason often reveals the weak point you can attack. A CO-50 frequently comes down to one of a few situations: the reviewer applied a general internal policy that does not fit your specific clinical picture, the plan never received the records that show what you already tried, the automated criteria flagged a missing step you in fact completed, or the reviewing physician was not in the same specialty as your treating doctor. Each of these is correctable. A denial built on an incomplete record falls apart the moment you supply the missing record, and a denial built on the wrong specialty's standard falls apart when a same-specialty letter contradicts it.

How to build the rebuttal

A strong CO-50 appeal rests on three pillars. Each is persuasive on its own. Together they are difficult to dismiss.

1. The treating physician's letter of medical necessity

This is usually the single most influential document in the appeal. A good letter does not simply assert that you need the treatment. It quotes the plan's own definition of medical necessity and walks through each prong, showing how your case meets it: the diagnosis with ICD-10 codes, the treatments already tried and their outcomes, the objective findings (imaging, labs, pathology, functional measures), and a clear statement that the requested service is the appropriate next step. See our full guide on how to write a letter of medical necessity for a worked template your physician can adapt.

2. Peer-reviewed evidence and specialty-society guidelines

Insurers respect the medical community's own consensus. Cite the clinical practice guidelines published by the relevant specialty society, and attach the peer-reviewed studies the guideline relies on, with PubMed identifiers. The right body depends on your condition:

• Oncology: the National Comprehensive Cancer Network (NCCN) guidelines, which many insurers explicitly reference in their own policies.
• Rheumatology: the American College of Rheumatology (ACR) treatment guidelines.
• Cardiology: the American Heart Association and American College of Cardiology (AHA/ACC) joint guidelines.
• Neurology: the American Academy of Neurology (AAN) practice guidelines.

When a national specialty society recommends the treatment your insurer called unnecessary, the reviewer has to explain why their internal criteria override the consensus of the physicians who treat your condition for a living. That is a difficult position to defend.

3. The insurer's own published medical policy

This is the most overlooked and often the most powerful pillar. Almost every insurer publishes medical policies, sometimes called coverage policies or clinical guidelines, that spell out exactly when a given service is considered medically necessary. Request the specific policy the insurer applied to your claim. Then compare your record against it line by line. If your case meets the criteria the insurer itself published, quote that policy back to them in your appeal. An insurer contradicting its own written standard is one of the strongest arguments you can make, because the reviewer cannot dismiss the source.

Get the exact criteria the insurer used to deny you

You do not have to guess what standard the insurer applied. Ask for it in writing. Request the medical-necessity criteria and clinical guidelines the plan used, the specialty of the reviewing physician, and the specific rationale for the denial.

If your coverage is through an employer plan governed by ERISA, this right is explicit. Under 29 CFR § 2560.503-1(m)(8), you are entitled to documents, records, and other information "relevant" to your claim, which the regulation defines to include any policy or guidance the plan relied on, whether or not it was relied on in making the determination. That covers the internal medical-necessity criteria, the clinical guidelines, and the basis for the reviewer's conclusion. Send the request to the appeals address, keep a copy, and note the date. Once you have the criteria, your appeal can target them precisely rather than arguing in the dark.

Reading the criteria carefully is worth the effort. Look for the exact thresholds the policy sets: a required number of failed conservative treatments, a specific lab value or imaging finding, a documented duration of symptoms, or a particular diagnosis code. Then match each requirement to a line in your medical record and cite it. If the policy lists five criteria and you meet all five, say so explicitly and point to the evidence for each. If the insurer applied a criterion that does not appear in its own published policy, that is itself grounds for appeal, because the plan must apply the standard it actually adopted.

Even if your plan is not governed by ERISA, similar disclosure rights usually exist under state law and ACA rules. The denial notice itself is required to state the specific reason for the denial and to tell you how to request the criteria used. If the notice is vague, that vagueness is worth raising, because a denial you cannot understand is a denial you cannot meaningfully appeal, and reviewers know it.

The appeal path: internal first, then external review

Appeals run in a defined sequence. You start with the insurer's internal appeal, in which the plan re-reviews its own decision. For ACA-governed and ERISA plans you generally have at least 180 days from the date of the denial notice to file, but check your denial letter for the exact deadline and file early. Submit your letter of medical necessity, the guidelines, the studies, and the comparison against the insurer's own policy together as a single, organized package.

Before or during the internal appeal, ask whether your physician can request a peer-to-peer review, a direct call between your treating doctor and the insurer's reviewing physician. These calls give your doctor a chance to explain the clinical reasoning in real time and to correct any misunderstanding about the record. A peer-to-peer does not replace the written appeal, but it can resolve a CO-50 quickly when the only problem was a reviewer who did not have the full picture. Whatever the outcome of the call, keep building the written record so you are ready to escalate.

If the internal appeal is denied, you move to external review. Under 45 CFR § 147.136 and 42 USC § 300gg-19, an Independent Review Organization assigns an independent physician reviewer who re-decides the medical-necessity question without any financial tie to your insurer. This is the step many patients never take, and it is precisely the step designed for CO-50 denials, because external review exists to resolve disputes about medical judgment. The insurer is bound by the IRO's decision. Our guide on how external review and IROs work walks through how to request it and what to send.

When your care is urgent: the 72-hour expedited path

If waiting for the standard timeline would seriously jeopardize your life, your health, or your ability to regain maximum function, you can request an expedited appeal. The plan must then decide as quickly as your medical condition requires, and generally no later than 72 hours after receiving the request. In urgent situations you can also request an expedited external review that runs at the same time as the internal appeal, so you are not forced to wait for one to finish before starting the other. When you request expedited handling, have your physician document the urgency in writing, because that clinical statement is what triggers the faster clock.

Putting it together

A CO-50 denial is the start of a conversation, not the end of one. Request the criteria the insurer applied, build the three-pillar rebuttal of physician letter, specialty-society evidence, and the insurer's own policy, file the internal appeal before the deadline, and carry the case to independent external review if needed. For a refresher on the broader process and a directory of denial codes, see our medical-necessity denial code reference or start a free appeal from the Counterclaim home page.