Guide · Payer letters

Medical necessity letters for therapy: a structure that documents, not decides

Last reviewed October 12, 2026 · 7 min read

A letter of medical necessity is a clinician's written summary, drawn from the record, of why a specific service is clinically appropriate for a specific client. The payer applies its own definition of medical necessity and decides coverage. Your letter documents the facts that definition asks about; it does not decide anything.

• Who decidesThe payer, under its own definition
• Who writesThe treating clinician
• SourceYour own record, dated
• ToneFactual, client-specific

How payers define medical necessity

For Medicare, the statute excludes items and services that "are not reasonable and necessary for the diagnosis or treatment of illness or injury or to improve the functioning of a malformed body member" (42 U.S.C. 1395y(a)(1)(A)). Commercial plans and Medicaid programs publish their own definitions in plan documents and medical policies, often with level-of-care criteria. Read the definition that applies to your client's plan and write to its elements.

A nine-part structure

1. Header. Client name, date of birth, member ID, date of the letter, payer and reference number if any.
2. Request. The service requested with CPT code, frequency and period, for example weekly 90837 for 12 weeks.
3. Clinician. Your name, license type, license number and state.
4. Diagnoses. ICD-10-CM codes and date of the evaluation, as documented.
5. Current presentation. Symptoms with concrete examples of functional impact at work, school, home or in relationships.
6. Treatment to date. Start date, sessions attended, interventions, and response with dated measure scores.
7. Plan. Goals and measurable objectives for the period requested.
8. Clinical rationale. In your own words, why this service at this frequency fits the client's current needs, linked to the payer's published criteria where they exist.
9. Signature. Signature, credentials, date and contact details.

Evidence beats adjectives

What to leave out

• Psychotherapy notes. Process notes kept separately from the record are defined in 45 CFR 164.501 and have extra protection. A letter does not need them.
• More than the payer needs. HIPAA's minimum necessary standard applies to disclosures for payment. Unverified Section 164.502(b) was not re-read on October 12, 2026.
• Promises. Statements about what treatment will achieve, or claims that the client will be harmed without it, unless they are your documented clinical view.
• Copy-paste. Identical letters for different clients read as templates and invite questions.

Need the plan behind the letter? See our guide to 90-day treatment plan updates.