Letter of Medical Necessity: How to Write One and Overturn a Denial

You went to the appointment. Your doctor said, “You need this — let's get it scheduled.” And then, weeks later, the letter arrived: Your insurance has determined that this service does not meet medical necessity. It is a frustrating sentence to read. It sounds final. It is not. In most situations, a “medical necessity” denial can be appealed — and a well-written Letter of Medical Necessity from your physician's office is the strongest part of that appeal.

What a Letter of Medical Necessity is (and isn't)

A Letter of Medical Necessity (sometimes abbreviated LMN, or called a letter of medical justification) is written by your treating clinician and explains in clinical language why a specific treatment, medication, test, device, or service is medically necessary for you — not in general, but in your particular situation.

It is not the same as a patient appeal letter (which you write yourself, often with the LMN attached). It is not a referral, and it is not a prior-authorization form. It is a clinical narrative that addresses the insurer's denial reason directly, in the language insurance reviewers are trained to look for. Two things to know up front: the letter is authored by a clinician — physician, advanced-practice provider, or qualified specialist — because insurers give weight to a clinical signature and credential; and you can absolutely help prepare a strong draft to bring to the office, which many practices appreciate.

Why this letter is worth the effort

Denials are common, and most are never appealed. According to a Kaiser Family Foundation (KFF) analysis of HealthCare.gov (ACA marketplace) data, insurers denied about 19% of in-network claims in 2024 — roughly one in five — and about 37% of out-of-network claims. Of the tens of millions of denied in-network claims that year, fewer than 1% were appealed by consumers, and of the appeals that were filed, insurers upheld about 66% of their original denials on internal review.

Two things follow. First, the appeal system exists, but very few people use it. Second, the quality of the appeal appears to matter: internal reviewers uphold most denials, but an independent external review (the step that follows an internal appeal) overturns a meaningful share. A 2025 Health Affairs study of external reviews in four states, cited by KFF, found that nearly half of all external-review decisions overturned the original denial; for cancer genetic testing specifically, about 30% were overturned. Specific, evidence-aligned letters tend to outperform generic ones — which is why this letter is worth the effort.

The seven sections every Letter of Medical Necessity should include

Formats vary by insurer and setting, but the components most consistently recommended in physician-organization guidance — including the American Academy of Family Physicians, the American Academy for Cerebral Palsy and Developmental Medicine, and the American Academy of Pediatrics — are remarkably consistent. Look for these seven.

1. Patient identification

The header should include your full name, date of birth, member or policy ID, the insurer's name, and the claim or prior-authorization reference number from the denial notice. Reviewers triage hundreds of letters; an identification mismatch can stop the review before it starts.

2. The specific request

State precisely what is being requested — the procedure, medication, test, or device — using the exact billing codes when available: the CPT code (procedures and services), HCPCS code (durable medical equipment, certain drugs), or NDC (drug national drug code), plus the ICD-10-CM diagnosis code(s) that justify it. Coding specificity is one of the most common gaps in unsuccessful appeals.

3. Diagnosis and clinical history

A concise summary of your relevant diagnoses, the timeline of symptoms and findings, prior workup (imaging, labs, consults), and how the current request fits the larger clinical picture. Specific, dated, sourced details are more persuasive than general statements.

4. Treatments already tried (or why they would be inappropriate)

Insurers often require evidence that less expensive or less invasive options were tried first (“step therapy” or “fail-first” requirements). The letter should list each prior treatment, when it was tried, and the outcome — and, if alternatives were skipped, the clinical reason (contraindication, prior adverse reaction, or a guideline-based exemption).

5. Clinical rationale tied to evidence

This is the heart of the letter: why this treatment, for this patient, now. Where possible, the rationale should cite clinical practice guidelines, FDA labeling, or peer-reviewed studies — and, ideally, the insurer's own medical-policy bulletin if one exists for this service. Citing the insurer's own policy by number, when applicable, is one of the highest-impact moves in an LMN.

6. Prognosis with and without the requested service

A short paragraph on the expected clinical course if the treatment is approved, and if it is denied or delayed. Keep this evidence-aligned and avoid speculation.

7. Provider attestation

The clinician's printed name, credentials, license number or NPI, signature, and contact information. Some letters also note board certifications and the clinician's role in your care (treating physician, specialist consultant, and so on).

What the letter looks like on the page

A finished Letter of Medical Necessity is usually a one- to two-page clinical letter on the practice's letterhead, addressed to the insurer's medical director or appeals department. The skeleton below is not words to copy — it is the shape of the page, so you know what to expect:

The shape on the page is short, dense, and specific. Insurance reviewers read dozens of these a day; a clean letter with concrete codes and a named medical policy outperforms a long, narrative one most of the time.

Five practical things to bring to your physician's office

If you are helping assemble the letter, these materials make the office's job easier — and the letter stronger:

• The full denial notice or Explanation of Benefits (EOB). This is where the denial code lives — for example, CO-50 (“not deemed a medical necessity by the payer”) or CO-167 (“this diagnosis is not covered”), per the X12 Claim Adjustment Reason Codes. The letter must address the specific denial reason. (Not sure what your EOB is telling you? The EOB Explainer decodes it. Appealing a GLP-1 denial — Wegovy, Zepbound, Ozempic, Mounjaro? See the GLP-1 Insurance Navigator.)
• Your insurer's medical-policy bulletin for the service, if one exists. Most major insurers publish these online; the letter should map your situation to the bulletin's listed criteria.
• A brief written timeline of treatments already tried, with dates, outcomes, and any side effects — this makes the step-therapy section accurate and complete.
• Relevant test results and imaging reports that support the request. The clinic likely has these, but a written list speeds cross-referencing.
• The appeal deadline from the denial notice. This is non-negotiable timing information (see below).

Different plans, different rules

Two patients with the same denial can have very different appeal options — and most people don't know which kind of plan they have until something goes wrong. Three categories are worth knowing.

Your denial notice should state which rules apply; if not, your insurance card or your employer's HR department can confirm. Knowing your plan type tells you which doors are open after the letter is submitted.

Where the letter fits in the appeal process

For most employer-based and individual-market plans, federal ERISA rules require plans to give you at least 180 days after an adverse benefit determination to file an internal appeal. Medicare Advantage is tighter — typically 60 days from the Notice of Denial of Medical Coverage. Your plan documents and state law may add rights or different windows, and the denial notice lists the deadline that applies to you.

Urgent? Ask for an expedited appeal

When a delay could seriously jeopardize life, health, or the ability to regain maximum function, most plans must offer an expedited appeal with a decision typically within 72 hours. This is built into ACA-compliant plans, Medicare Advantage, and most ERISA-governed employer plans. If your situation is urgent, ask the office to label the appeal expedited and explain why.

Internal review, then external review

The appeal usually has two phases: an internal review by the insurer (the LMN goes here, attached to a patient appeal letter); and, if upheld, an external review by an independent reviewer. Where the plan's medical judgment is at issue, federal rules require the appeal be evaluated by a health professional with appropriate training in the relevant field — not the original denier. For non-grandfathered plans, federal rules give you at least four months after a final internal denial to request external review; the federal process is administered by MAXIMUS Federal Services where a state external-review process does not apply.

When appeals are exhausted

If both internal and external reviews uphold the denial, more pathways exist. Fully-insured plans: file a complaint with your state insurance commissioner. ERISA self-funded plans: complaints go to the U.S. Department of Labor's Employee Benefits Security Administration (EBSA). Medicare Advantage and Medicaid managed care: the pathway continues through CMS, including administrative-law-judge review. Many state attorneys general also accept consumer health-insurance complaints.

Language access

Federal rules require plan notices about denials and appeal rights to be available in non-English languages spoken by 10% or more of the population in a given county. If your denial notice or appeal materials are not in your primary language, ask the plan for a translation — it should be available at no cost.

The practical takeaway: appeal windows are real, often short, and listed on the denial notice. Read that notice the day it arrives, calendar the deadline, and do not wait for a follow-up bill before starting the letter.