HealthcareFree tool
Medical Necessity Letter From Criteria
A medical necessity letter built from the plan's criteria: each one answered in order, with the dated chart entry behind it, and every gap flagged.
Free to start · Opens in your River workspace, ready to edit and share
How to write a letter of medical necessity
A letter of medical necessity asks a health plan to cover a specific service, drug or device for one patient, and shows that the patient meets the plan's coverage criteria. A strong one identifies the patient and the request, then answers each criterion in the plan's own order. Every answer points to a dated entry in the medical record, and the treating clinician signs it.
Write it for a reviewer working through a checklist, not for a jury. Under federal rules for Medicare Advantage, one permitted use of prior authorization is to confirm the presence of diagnoses or other medical criteria1 that coverage depends on. Your letter's job is to make each one easy to find.
Get the plan's criteria first
Start from the criteria the reviewer will apply to this service, not from a sample letter. A Medicare Advantage plan must follow Medicare's national and local coverage determinations, which you can look up in CMS's Medicare Coverage Database. Any internal criteria it adds must be publicly accessible2.
Most commercial plans post their medical policies online. If the plan has already asked for more information or denied the request, start with the criterion its notice names.
Then turn the policy into a numbered list:
- One requirement per line, in the plan's order and numbering
- The plan's exact words, quoted rather than paraphrased
- Compound criteria split apart: "3 months of methotrexate" needs a dose, start and stop dates, and a result
- Exclusions listed on their own, since each needs evidence that it doesn't apply
- The policy's date or version noted, since plans revise them
Match each criterion to the record
For each line, find the note, result or record that answers it, with its date, author and exact words. The medical equipment chapter of Medicare's Program Integrity Manual says a physician's order or attestation does not, by itself, document medical necessity3, even when signed. The patient's medical record has to support it.
The same chapter lists what the record should show, including the diagnosis, its course, functional limits, and other treatments tried and their results.
Common criteria and the evidence that answers them
| Criterion | Evidence that answers it | Common near miss |
|---|---|---|
| Diagnosis | A dated note that documents it, with the ICD-10 code | The code is on the order but in no note |
| Severity | Scores, lab values or imaging, with dates and units | A result older than the policy's time window |
| Prior treatment | Each drug or therapy, with dose, start and stop dates, and result | "Failed methotrexate," with no dose or dates |
| Function | Measured limits from an exam or therapy evaluation | "Struggles at home," with no measurement |
| Prescriber and setting | The ordering clinician's specialty and the site of care | A specialist is required, but primary care placed the order |
| Exclusions | A dated entry showing none applies | A required screening test ordered, with no result on file |
Near misses are easy to overlook and cheap to fix. Whatever you cite, point the reviewer to the exact page. In a 2022 report on 15 of the largest Medicare Advantage insurers, HHS's Office of Inspector General estimated that 13% of their prior authorization denials met Medicare coverage rules4. One common cause: plans said requests lacked enough documentation when the existing records were sufficient.
For a continuation request, show measured progress since the approval began. A treatment plan with measurable goals gives you numbers to cite.
Close the gaps before you write
If a criterion has nothing behind it, don't write around it. Look for the evidence elsewhere first: outside records, pharmacy fill history, or results sent to another office. If the clinician knows something the chart doesn't say, ask them to document it in a new note.
Never backdate. Any addendum should show the date it was written and who wrote it. If a criterion truly doesn't fit this patient, such as a required first drug they can't take, say so and cite the note that explains why.
The clinical case belongs to the clinician
Staff can find and arrange the evidence, but every clinical statement must come from the treating clinician, who reviews and signs the letter. This guide is general information about US health plans, not legal or medical advice. Involve your compliance officer if a plan questions a record's accuracy.
What goes in a letter of medical necessity
Identify the request
Patient name, date of birth and member ID; the service and its CPT or HCPCS code; diagnosis codes; site of care; and the ordering clinician's NPI.
Ask in one sentence
Say what you want approved, how much and for how long, before any argument.
Answer each criterion
One numbered section per criterion, in the plan's order: the criterion quoted, the answer, then the source and date.
List the enclosures
Number them to match the answers, so the reviewer can find each page.
Sign it
The treating clinician signs, with credentials and a direct number for a peer-to-peer call.
Let the record do the arguing. Medicare Advantage plans must consider the patient's medical history, physician recommendations and clinical notes2 when deciding medical necessity. A dated quote from a note tells the reviewer more than an adjective like "severe" or "refractory."
Criterion 2: "Inadequate response to at least 3 months of methotrexate."
Met. Methotrexate 20 mg weekly from March 3 to September 8, 2026, 27 weeks in all. Disease activity stayed high (a CDAI score of 31 on September 8). Source: rheumatology note, September 8, 2026 (Enclosure 2).
Criterion 4: "Negative tuberculosis screening within the past 12 months."
Not yet documented. A screening test was ordered on October 2, 2026, and the result will follow as Enclosure 5.
- Quoted, not paraphrased. The reviewer can check each answer against the plan's own words.
- Numbers from the chart. Dose, dates and score come from a dated note, sent as an enclosure.
- The gap stays visible. Saying what is missing beats a vague sentence that hides it.
Send it on time and follow up
Since 2026, a CMS rule has required Medicare Advantage, Medicaid and CHIP plans to decide standard requests within 7 calendar days and expedited ones within 72 hours5. The same rule requires a specific reason for every denial. Employer plans covered by ERISA have 15 days, or 72 hours for urgent care6.
Prior authorization decision deadlines in the US
| Plan type | Standard request | Expedited request |
|---|---|---|
| Medicare Advantage, Medicaid and CHIP | 7 calendar days, extendable by up to 14 in limited cases | 72 hours |
| Employer plans covered by ERISA | 15 days, extendable once by up to 15 | 72 hours |
| Other plans | Check the plan documents and state law | Check the plan documents and state law |
The CMS limits don't apply to drugs, which follow separate rules. Each clock starts when the plan receives the request, so keep proof of that date.
Ask for an expedited decision when the standard wait could seriously jeopardize the patient's life, health or ability to regain maximum function. Employer plans also count severe pain that, in a physician's opinion, can't be adequately managed without the care. They must treat a request as urgent when a physician with knowledge of the patient's condition determines it is6.
Before you send
- Every criterion is answered, in the plan's order and numbering
- Each answer cites a dated entry, and that entry is enclosed
- Gaps are stated plainly, not written around
- Codes, quantities and dates match the order
- The treating clinician has reviewed and signed it
- It's marked expedited only if waiting could seriously harm the patient
If the plan denies the request, answer the reason it gives. KFF, the health policy research group, found that in 2024 Medicare Advantage plans fully or partly overturned 80.7% of the prior authorization denials that were appealed7. Yet only 11.5% of denials were appealed. A prior authorization appeal letter answers a denial point by point.
A prior authorization request template helps you track what each payer asks for. To draft the letter itself, paste the criteria and the clinician's notes into the tool at the top of this page. It matches each criterion to the record and marks what's missing.
How it works
Paste the plan's coverage criteria and the clinician's notes, and choose what the letter is for: a first request, a resubmission, an appeal, a continuation or an expedited request. River matches each criterion to the dated line in the record that answers it. Then it writes the letter in the plan's order and numbering, with the source and date under every answer.
River writes no clinical content of its own. A criterion with nothing behind it is marked unsupported, never filled in, and a near miss, such as a drug trial with no dates, is named exactly. You also get the criterion map as a sheet and a short list for the clinician to close before signing, all in your River workspace.
Paste criteria and notes
Add the plan's coverage criteria and the clinician's documentation, then choose what the letter is for.
River maps the record
It matches each criterion to the dated line that answers it and names every near miss.
Get your letter
A letter in the plan's order and numbering, plus a sheet with one row per criterion.
Finish and sign
Edit anything, have the clinician close the gaps and sign, or ask River for peer-to-peer talking points.
What you get
- One numbered section per coverage criterion, in the plan's own order, numbering and wording
- Each criterion quoted, then answered by a dated line from the record and the document it came from
- Near misses named precisely, like a drug trial without dates or a lab value in the wrong unit
- Criteria with no evidence behind them marked unsupported, never filled in with plausible-sounding prose
- A sheet with one row per criterion: the evidence, its source and date, and whether it is met
- A to-do list for the clinician, quickest fixes first, plus the decision deadline or where to find it
Common questions
What is a letter of medical necessity?
A letter of medical necessity is the treating clinician's signed explanation of why a patient needs a specific treatment, showing that it meets the health plan's coverage criteria. Plans ask for one with prior authorization requests, appeals and exception requests, such as covering a drug that isn't on the plan's formulary. The reviewer checks it against the medical record.
Who should write a letter of medical necessity?
The treating clinician should sign it, because it vouches for the care of a patient they treat. Authorization or billing staff often do the groundwork: they pull the plan's criteria, find the chart entries that answer each one and draft the structure. The clinician then checks every clinical statement, adds what only they know, and signs.
How long should the letter be?
Long enough to answer every criterion, and no longer. That is often one or two pages: an identifying header, a one-sentence request and a short numbered answer per criterion, with the detail left in the enclosed records. Reviewers check criteria one at a time, so a long narrative only makes each answer harder to find.
Does a good letter guarantee approval?
No. The plan decides by applying its coverage criteria to the patient's record, and a letter can't supply evidence the chart doesn't hold. A clear letter makes each criterion quick to verify, which helps avoid requests for more information and denials over documentation a reviewer missed. If a denial still comes, its stated reason tells you what to answer.
Can I paste real patient details?
Follow your organization's rules for sharing patient information with outside tools. If they don't allow it, remove names, member IDs and other identifiers before you paste. River uses the identifiers you supply exactly as given, and where you removed them it keeps placeholders such as [Patient name] and [Member ID] for you to fill in later.
How do I write a letter of medical necessity with River?
Paste the plan's coverage criteria and the clinician's notes into the box at the top of this page, and choose what the letter is for. River matches each criterion to the record, writes the letter in the plan's order and lists what's missing. Everything opens in your free River workspace, ready for the clinician to check and sign.
Is the medical necessity letter tool free?
Yes, it is free to start. Your letter and its evidence map open in a River workspace, where you can edit them, share them with the clinician or ask River to change them. Paid plans add more AI use and access to the most advanced models.
Sources
eCFR. “42 CFR 422.138 — Prior authorization.” ecfr.gov (opens in a new tab)
eCFR. “42 CFR 422.101 — Requirements relating to basic benefits.” ecfr.gov (opens in a new tab)
CMS. “Medicare Program Integrity Manual, Chapter 5 – Durable Medical Equipment, Prosthetics, Orthotics, and Supplies (DMEPOS) Items and Services Having Special DME Review Considerations.” cms.gov (opens in a new tab)
HHS Office of Inspector General. “Some Medicare Advantage Organization Denials of Prior Authorization Requests Raise Concerns About Beneficiary Access to Medically Necessary Care.” oig.hhs.gov (opens in a new tab)
CMS. “CMS Interoperability and Prior Authorization Final Rule CMS-0057-F.” cms.gov (opens in a new tab)
eCFR. “29 CFR 2560.503-1: Claims procedure.” ecfr.gov (opens in a new tab)
KFF. “Medicare Advantage Insurers Made Nearly 53 Million Prior Authorization Determinations in 2024.” kff.org (opens in a new tab)
Built by
Chandler Supple
Co-founder & CEO, River
Chandler has spent the last six years working in AI, building tools for millions of users and Fortune 100 companies. In 2025, he co-founded River to build a new kind of operating system for AI agents.