Written and maintained by CASRAI Editorial Board
Last updated
General information, not medical, legal or coverage advice. Your plan’s own documents and your clinician govern your situation.
Short answer
A letter of medical necessity (LMN) is a letter from a patient’s doctor or other provider that explains why a service or item is medically necessary, written when a health insurer asks for more proof than the prescription or order. Insurers typically request it during certification or utilization review, and read it alongside medical records and the plan’s medical policy (NAIC, consumer sheet dated August 2021).
What is a letter of medical necessity?
The NAIC explains that a policy defines “medically necessary,” but the term usually means health care a prudent physician, using professional standards and judgment, would give a patient. It states that “a doctor’s prescription or order for a service is the first evidence of medical necessity,” and that an insurer wanting more proof may ask the provider for a “Letter of Medical Necessity.” This page covers that commercial and private-plan document. For the Medicare form used for durable medical equipment, see what a CMN is and what replaced it. Searchers also write “medical letter of necessity”; the document is the same.
Who writes it and how to get a letter of medical necessity (LMN)
Can I ask my doctor for a letter of medical necessity?
Yes. The NAIC sheet says the insurer may ask your doctor or other provider for the letter, so the provider who ordered the service is the usual author. Whether a provider agrees to write one, and what they conclude, is a clinical decision for them.
How to get a letter of medical necessity (LMN)
The request normally comes through the insurer’s certification or utilization review process, so the starting point is the plan’s request or notice, which says what the insurer wants to see. The provider then prepares the letter and sends it with supporting records.
Where the letter sits in insurer review
The NAIC describes three points at which an insurer can review medical necessity. The letter can be part of any of them.
| Review type | When it happens | What the insurer does (NAIC) |
|---|---|---|
| Precertification | Before the treatment is provided | Typically reviews the letter, medical records and the plan’s medical policy |
| Concurrent review | While treatment is ongoing | Decides whether the treatment is medically necessary |
| Retrospective review | After services were provided | Decides whether services were medically necessary or, for emergency services, truly required emergency care |
For how precertification differs from prior authorization, see prior authorization vs precertification. State rules on review are covered in state utilization review laws.
How do you document medical necessity? A reviewer checklist
The NAIC lists the criteria a typical definition of medical necessity includes. A letter that speaks to each one gives a reviewer what the definition asks for. The plan’s own definition governs, so check it first.
| Typical criterion (NAIC) | What a reviewer is looking to see addressed |
|---|---|
| Evaluates, diagnoses or treats an illness, injury, disease or its symptoms | The condition and how the service relates to it |
| Follows generally accepted standards of medical practice | That the service is consistent with accepted practice |
| Clinically appropriate | Why it suits this patient’s circumstances |
| Not primarily for the convenience of the patient, provider or family | That the reason is clinical rather than convenience |
| Not costlier than an at-least-as-effective alternative | Why alternatives are not equally effective, where they exist |
| Not experimental, investigational or cosmetic | That the service is not in an excluded category under the plan’s guidelines |
The second column is an editorial reading of what each criterion asks for, not wording from the NAIC. The letter works alongside, not instead of, the order and the medical records. For the equipment side of this documentation, see Medicare DME coverage and DME HCPCS codes.
When coverage is denied: rights under ERISA group plans
For employer group health plans covered by the federal ERISA claims-procedure regulation (29 CFR 2560.503-1), a denial after review carries specific rights. These rules apply to ERISA group health plans; individual-market, Medicare and state-regulated coverage follow other rules.
- Guideline copy. If the denial relied on an internal rule, guideline or protocol, the notice must include it or say it was relied on and that a copy will be provided free of charge on request (paragraph (g)(1)(v)(A)).
- Clinical explanation. For a medical necessity or experimental-treatment denial, the notice must explain the scientific or clinical judgment, applying the plan’s terms to the claimant’s circumstances, or state that it is available free on request (paragraph (g)(1)(v)(B)).
- Appeal window. The plan must allow at least 180 days after receiving the denial notice to appeal (paragraph (h)(3)(i)).
- Documents. The claimant may request, free of charge, reasonable access to and copies of all documents, records and other information relevant to the claim (paragraph (h)(2)(iii)).
The NAIC adds that an insurer must make its medical guidelines available if it used them to deny coverage. A letter written after a denial can respond to the stated reason.
Where it matters for labs, clinics and supply decisions
Clinic and billing staff handle these requests when a plan questions a service or an item of equipment or supplies. Because the plan’s medical policy and definition decide the outcome, staff do well to obtain them before the provider drafts anything. The denial notice is the document that names the guideline relied on.
Common misunderstandings
- “The letter guarantees approval.” It does not. The insurer decides against its own definition, medical policy and records.
- “It replaces the prescription.” The NAIC treats the order as the first evidence; the letter is further proof when requested.
- “It is the same as the Medicare CMN.” The CMN is a Medicare DME form; see What Is a CMN?
- “Medical necessity means the same everywhere.” The NAIC says the policy defines it; the wording differs by plan.
FAQ
What is a letter of medical necessity?
A provider’s letter explaining why a service is medically necessary, requested by an insurer that wants more than the prescription or order as proof (NAIC).
Can I ask my doctor for a letter of medical necessity?
Yes. The provider who ordered the service is the usual author, and they decide whether and how to write it.
How do you document medical necessity?
Begin with the order and medical records, then have the letter address the plan’s own definition, using the checklist above as a guide.
How long is a letter of medical necessity valid?
That is set by the insurer or plan, so check the plan documents or the approval notice.
How to get a letter of medical necessity (LMN)?
Follow the insurer’s request or denial notice, and ask the ordering provider to prepare the letter with supporting records.
Sources
- NAIC, Health Care Bills: Understanding Medical Necessity (consumer sheet dated August 2021; checked 2026-11-01).
- 29 CFR 2560.503-1, Claims procedure (Cornell LII) (checked 2026-11-01).
