Letter of Medical Necessity: How to Write One

A letter of medical necessity (LMN) is a document a clinician writes to explain why a specific treatment, medication, device, or service is medically necessary for a particular patient. Payers, insurers, and durable medical equipment suppliers request it to justify coverage, especially when a request falls outside standard criteria or has been denied. A strong letter connects the patient's diagnosis to the requested item and shows why alternatives are not appropriate.

What Is a Letter of Medical Necessity?

A letter of medical necessity is a written statement from a treating clinician that documents the clinical reasons a service or item is required. It is most often used for durable medical equipment (such as a wheelchair or CPAP), non-formulary or high-cost medications, specialized therapies, and services that require prior authorization or appeal after a denial.

The letter does not replace the medical record. Medicare and other payers expect the record itself to support medical necessity, and a letter that is not backed by the chart carries little weight. As CMS explains in its guidance on complying with medical record documentation requirements, documentation must support the medical necessity of the services billed. The letter summarizes and points to that record; it does not stand in for it.

When Is a Letter of Medical Necessity Required?

A clinician is typically asked for a letter of medical necessity in situations such as:

• Durable medical equipment (DME): wheelchairs, hospital beds, CPAP machines, prosthetics, and similar items.
• Non-formulary or step-therapy medications: when a payer requires justification to cover a drug outside its preferred list.
• Prior authorization: as supporting documentation for a request the payer must approve before the service. This overlaps closely with prior authorization documentation.
• Appeals: to overturn a denial, where the letter addresses the specific reason the request was rejected.
• Specialized services or therapies: that fall outside routine coverage criteria.

What to Include in a Letter of Medical Necessity

A complete letter answers, in the payer's terms, why this patient needs this item now. The elements below are the ones reviewers look for.

1. Patient and provider identifiers: patient name, date of birth, insurance ID, and the ordering clinician's name, credentials, and contact information.
2. Diagnosis: the relevant condition, ideally with the ICD-10 code, stated clearly.
3. The requested item or service: named specifically, with any codes (such as HCPCS or CPT) where applicable.
4. Clinical justification: how the patient's condition, symptoms, and functional limitations create the need for the item.
5. Treatments already tried: what has been attempted and why it was insufficient, which is often the deciding factor.
6. Expected benefit: what the item is intended to achieve for this patient.
7. Supporting evidence: relevant exam findings, test results, or guideline references from the record.
8. Signature and date: the treating clinician's authenticated signature.

Letter of Medical Necessity: Structure at a Glance

Tips for Writing a Strong Letter

• Address the payer's specific criteria or the exact reason for a denial, rather than writing a general statement.
• Be concrete about function: describe what the patient cannot do without the item, not only the diagnosis.
• Document prior treatments and their outcomes, since payers frequently require evidence that alternatives were tried.
• Keep the letter consistent with the medical record; contradictions weaken the request.
• Use plain, specific language and avoid unsupported superlatives.

Common Mistakes to Avoid

• Stating the diagnosis without connecting it to the specific need for the item.
• Omitting the history of failed or inadequate alternatives.
• Writing a letter that is not supported by the underlying chart.
• Leaving out identifiers or codes the payer needs to process the request.

How an AI Scribe Helps With Letters of Medical Necessity

The hardest part of a medical necessity letter is usually not the format; it is pulling together the clinical detail that justifies the request. That detail lives in the visit documentation. DocuMed AI is a HIPAA-compliant AI medical scribe that captures the encounter and drafts a structured note, so the diagnosis, history, prior treatments, and findings are recorded accurately at the point of care.

From that base note, the DocuMed AI Copilot can help draft related correspondence, in the same way it can generate a referral letter from a completed note. The clinician reviews and edits every draft, confirms it matches the record, and adds their signature before it is sent, since a letter of medical necessity is a formal attestation. For more on automating clinical correspondence, see this overview of clinical letter automation, or request a demo to see the workflow. Accurate coding in the underlying note also matters here; see this guide to E/M coding and documentation.

Frequently Asked Questions

Who writes a letter of medical necessity?

The treating clinician writes and signs it, because it is a professional attestation that the requested item or service is medically necessary for that patient. Staff may help assemble supporting information, but the clinician is responsible for the content and signature.

Does a letter of medical necessity guarantee coverage?

No. It supports a coverage request, but the payer makes the decision based on its own criteria and the supporting documentation. A well-written letter that addresses the payer's criteria improves the chance of approval but does not guarantee it.

What is the difference between a letter of medical necessity and prior authorization?

Prior authorization is the payer's approval process for a service; a letter of medical necessity is one document that can support that process. See prior authorization documentation for how the two fit together.

Can an AI scribe write the letter for me?

An AI scribe can draft a letter from your visit documentation, but the clinician must review it, confirm it matches the record, and sign it. The letter is an attestation, so the responsibility stays with the treating clinician.