The CGM Letter of Medical Necessity: What Wins Coverage and Why Thin Ones Get Denied

The denial letter says the CGM is "not medically necessary." You read that phrase standing in the kitchen and it lands like an accusation: your 3am lows, the shaking hands in the grocery store, the A1C that won't settle, all waved off by someone who has never met you. Here is the reframe that wins this fight: the reviewer never judged your body. They judged a packet. And the document that packet lives or dies on is the letter of medical necessity (LMN). Most CGM denials trace back to a thin one, and a thin letter is fixable in a week.

When do you actually need a letter of medical necessity?

Two moments. First, when your plan requires prior authorization for a CGM, which most commercial plans do: the LMN (or a chart note doing the same job) is the core of the packet your prescriber submits. Second, after a denial on medical-necessity grounds: the appeal is essentially a stronger LMN plus the evidence the first packet lacked. If your plan covers CGMs without prior auth and your claim sailed through, you never see this document. The moment a reviewer has to be convinced, the LMN is the whole argument. Before anything gets written, call the plan and ask for its clinical policy for continuous glucose monitors; they must provide it, and a letter written to the plan's actual checklist beats a generic one every time. The broader approval machinery (channels, checklists, manufacturer bridge programs) is covered in the CGM coverage guide.

Medicare's real CGM criteria (simpler than the denial implies)

Medicare covers CGMs under Part B as durable medical equipment, and since the 2023 rule change the criteria are short:

• You have diabetes and are treated with insulin. Any insulin, any dose, injections or pump. There is no minimum number of daily injections anymore.
• Or you have a history of problematic hypoglycemia. The coverage determination spells it out: more than one level 2 event (glucose under 54 mg/dL) despite treatment adjustments, or one level 3 event severe enough to need someone else's help.
• You saw the prescriber within the six months before the order, in person or by Medicare-approved telehealth, and every six months after to keep supplies flowing. This visit clock is the quiet killer of otherwise valid claims.
• The old testing-frequency rule is gone. Medicare retired the requirement to document four or more fingersticks a day, and the 2023 change opened coverage to every insulin user. A denial citing fingerstick counts is citing a rule that no longer exists, which makes it very appealable.
• Medicare Advantage must meet the same floor. MA plans must cover at least what Original Medicare covers, so an MA denial that contradicts these criteria is built to be appealed. Commercial plans write their own criteria, but most track Medicare's, and the ADA's Medicare CGM FAQ is a clean reference to cite when a reviewer gets the rules wrong. The rest of what Medicare covers for diabetes is in the Medicare diabetes guide.

What a strong letter of medical necessity contains

A weak LMN says "patient has diabetes and would benefit from CGM." A strong one reads like a case file. The elements reviewers look for:

• The diagnosis with its ICD-10 code. Type 1, type 2, or another form, coded precisely (E10.x, E11.x). A vague or mismatched code is a common silent denial cause, and the fix is clerical.
• The treatment regimen, exactly. Which insulin, what doses, injections or pump, plus other glucose-lowering medications. "Multiple daily injections of insulin aspart and glargine" approves; "on insulin" invites questions.
• Why fingersticks are insufficient for this patient. This is the heart of the letter. Hypoglycemia unawareness (lows arrive without warning symptoms), nocturnal lows a fingerstick schedule cannot catch, wide unexplained swings, a job or driving safety issue. Generic language fails here; your specifics win.
• The documented episodes, with dates. "Level 2 hypoglycemia (48 mg/dL) on March 3 and March 19 despite dose adjustment on March 5" is evidence. "Frequent lows" is an adjective. Every episode named in the letter should also exist in the chart.
• What was tried first. Structured fingerstick testing, dose adjustments, education. Showing the cheaper path was tried and failed answers the reviewer's next objection before it's raised.
• How the CGM changes management. Alarms for the unfelt lows, overnight data, trend-based dose decisions with the care team. One or two sentences connecting the device to the documented problem.

Who writes it, and what your job is. The prescriber writes and signs the LMN; a letter from you carries no clinical weight. But prescribers write these between appointments, from whatever the chart holds, and a chart that never heard about your 2am lows produces a letter that never mentions them. Your job is handing them the receipts. The lows you logged with dates and numbers, the swings, the symptoms, the times a fingerstick missed what was happening: that log IS the evidence the letter needs. Bring it as one clean page, ask that the episodes be added to the chart, and ask the office to write the letter to the plan's own criteria (bring the plan's CGM policy with you). Five minutes of your prep converts a form letter into an approvable one. The same receipts matter if you're also pursuing pump coverage, where the documentation bar is higher still.

Denied anyway? The appeal path

1. Read the denial for the named criterion. Somewhere in the letter is the specific box the reviewer marked empty: no recent visit, no documented regimen, no hypoglycemia evidence, wrong code. That word is your to-do list.
2. Resubmit the prior auth with the strengthened LMN first. If the denial named a fixable gap, a fresh submission with the missing documentation is usually faster than a formal appeal.
3. If the resubmission fails, file the internal appeal. Attach the strengthened LMN, the dated episode log, and a line quoting the plan's own published criteria back at it. Plans must decide pre-service appeals within 30 days; the full playbook is in the appeal guide.
4. Then external review. An independent clinical reviewer, free to you, and the plan is bound by the result. Denials that contradict the plan's written CGM policy, or Medicare's criteria on an MA plan, do badly there.
5. Bridge the gap meanwhile. Manufacturer copay programs and free-first-sensor offers keep a sensor on your arm during the fight, so the timeline pressure stays on the plan instead of on you.