A letter of medical necessity (LMN) is a formal document, typically written by your doctor, that explains to your insurance company why a specific treatment, procedure, or piece of equipment is essential for your health. It’s the single most important document in getting coverage for something your insurer has questioned or denied. The letter needs to connect your diagnosis directly to the requested treatment, demonstrate that alternatives have been tried or won’t work, and include enough clinical detail to satisfy a reviewer who has never met you.
Whether you’re seeking approval for surgery, a wheelchair, home health care, or a specialized therapy, the structure and strategy are largely the same. Here’s how to build one that gets approved.
Who Writes and Signs the Letter
Your treating physician writes and signs the letter. This is the doctor who has directly examined you, diagnosed your condition, and is prescribing the treatment or equipment in question. Insurance reviewers give far more weight to a letter from the physician who manages your care than to one from a provider with limited involvement. If specialists have contributed to your diagnosis or treatment plan, their evaluations can be attached as supporting documentation, but the primary letter should come from the prescribing doctor.
For durable medical equipment like wheelchairs or CPAP machines, Medicare and many private insurers require that the prescribing practitioner has had a face-to-face encounter with you within six months of the order. That visit must be documented in your medical record with both subjective complaints and objective findings. If this step is skipped, the claim can be denied on a technicality before anyone even reads the letter.
What Every Letter Must Include
Regardless of what you’re requesting, a strong LMN follows a predictable structure. Insurance reviewers process these quickly, so the information should be easy to find and logically organized.
- Header information: Your doctor’s name, practice, and contact details. The insurance company’s name and address. Your full name, date of birth, group number, and member ID.
- Diagnosis with codes: Your specific condition, stated in plain language and accompanied by the correct ICD-10 diagnostic code. Using the right code matters because insurers match it against their internal coverage policies.
- Clinical history: A summary of how long you’ve had the condition, how it has progressed, and how it affects your daily functioning. This should include objective findings from exams, imaging, or lab work, not just symptoms you’ve reported.
- What’s being requested: A clear description of the treatment, service, or equipment, including the specific procedure code if applicable, the quantity, and the expected duration.
- Medical rationale: The core of the letter. This section explains why this particular intervention is necessary for your accepted condition, supported by clinical evidence.
- Previous treatments tried: A record of what you’ve already attempted and why it didn’t work or isn’t sufficient.
The letter closes with a direct request for pre-authorization or coverage approval, followed by the physician’s signature, credentials, and NPI (National Provider Identifier) number.
Building the Medical Rationale
This is where most letters succeed or fail. The rationale section must do three things: explain the specific medical value of the requested treatment in relation to your condition, explain why standard or less costly alternatives aren’t sufficient, and explain how the treatment will cure, relieve, or reduce the severity or duration of your condition.
Vague language like “the patient would benefit from this treatment” gives a reviewer nothing to work with. Specifics are what persuade. If your doctor is recommending a particular therapy for chronic joint pain, for instance, the letter should reference the imaging that confirmed joint degeneration and clinical notes showing that anti-inflammatory medications failed to provide relief. Each claim should trace back to something documented in your chart.
Supporting the rationale with published clinical evidence strengthens the case considerably. Your doctor can reference clinical guidelines from professional medical societies or peer-reviewed research demonstrating that the requested treatment is effective for your specific diagnosis. This is especially important for newer therapies or anything the insurer might classify as “investigational,” since Medicare and most private plans limit coverage to items and services that are “reasonable and necessary for the diagnosis or treatment of an illness or injury.”
Documenting Failed Treatments
One of the most common reasons insurers deny claims is that the letter doesn’t demonstrate that cheaper or more conservative options were tried first. Insurance companies follow a step-therapy logic: they want to see that you started with first-line treatments before moving to something more expensive or invasive.
Your letter should list each previous treatment attempt with specific details. A sample letter from the Georgia Regents Weight Loss Center illustrates this well for bariatric surgery: it documents the patient’s use of diet pills (with duration and pounds lost), physician-supervised diets (with outcomes), commercial programs, and exercise limitations caused by co-existing conditions. Each attempt is paired with its result, showing a clear pattern of failure that justifies the surgical request.
The same principle applies to any request. If you need a power wheelchair, the letter should explain why a manual chair is no longer adequate. If you need home health care, it should clarify why outpatient visits can’t meet your needs. Always answer the implicit question the reviewer is asking: why not something less?
Special Requirements for Equipment
Letters requesting durable medical equipment like wheelchairs, hospital beds, prosthetics, or orthotics carry additional documentation requirements. CMS requires a Standard Written Order that includes your name or Medicare ID, the order date, a description of the item (which can be a general description, a billing code, or a brand and model number), the quantity, and the treating practitioner’s name, NPI, and signature.
For certain equipment categories, the written order must be communicated to the supplier before delivery. Your medical record must contain enough documentation to substantiate the necessity for both the type and quantity of items ordered, and for how frequently they’ll need to be used or replaced. Suppliers are required to keep this documentation on file for seven years, so thoroughness at this stage protects you if the claim is audited later.
For prosthetics and orthotics specifically, documentation created by the orthotist or prosthetist is considered part of your medical record and can be used alongside your physician’s notes to support the case for medical necessity.
Home Health Care Letters
Requesting in-home care requires a more detailed letter than most other services because insurers will question why the care can’t be delivered in an outpatient facility. The letter must specify the exact level of care needed (skilled nursing versus non-skilled aide), the number of hours per day and days per week for each level, and the specific duties each caregiver will perform over the next 180 days.
Your doctor also needs to document your current treatment, specific physical limitations based on objective medical evidence, and any effects that non-covered conditions have on your need for services. A physical exam must have been performed within 60 days of the request. If showing that home care would be less expensive than hospitalization while still meeting your medical needs, that cost comparison can be a persuasive addition to the letter.
Formatting for Readability
Insurance reviewers read dozens of these letters. A well-organized letter on practice letterhead with clearly labeled sections gets a fairer read than a dense narrative paragraph. Use a standard business letter format with the date, insurer address, and patient reference line at the top. Break the body into short, labeled sections: Diagnosis, Clinical History, Previous Treatments, Medical Rationale, and Request.
Attach supporting documents rather than trying to squeeze everything into the letter itself. Relevant attachments include diagnostic imaging reports, specialist evaluations, treatment records showing failed therapies, and copies of published clinical guidelines supporting the requested intervention. Reference these attachments within the letter so the reviewer knows they exist and where to find the relevant detail.
What to Do if the Claim Is Denied
A denial isn’t the end. Insurance companies frequently deny initial requests when the letter doesn’t meet their internal medical policies, which outline specific requirements for covering certain treatments. The most common reasons for denial include insufficient documentation that the treatment is medically necessary, failure to show that alternatives were tried, and missing clinical evidence linking the treatment to the diagnosis.
If your claim is denied, review the denial letter carefully. It will typically cite the specific policy criteria you didn’t meet. This tells you exactly what the appeal letter needs to address. Your doctor can then write a more targeted letter filling in the gaps, attaching additional records or research as needed.
Timelines for appeals vary by payer. For Medicare, the review of submitted medical records generally takes up to 20 calendar days, and appeal requests typically must be filed within 15 business days of the decision. Private insurers set their own deadlines, which are stated in the denial letter. Acting quickly matters because missing the appeal window can mean starting the entire process over.
Washington State’s Office of the Insurance Commissioner notes that insurers must cover mental health and substance abuse services the same way they cover medical and surgical ones, and generally cannot deny medically necessary gender-affirming care. If your denial falls into one of these categories, citing the applicable parity requirements in your appeal can carry significant weight.