What Is Medical Necessity?

Medical necessity is the standard health insurers use to determine whether a treatment, procedure, or service is appropriate and covered under your plan. In simple terms: if your insurer decides something is not medically necessary, they will not pay for it — even if your doctor prescribed it and you need it. Medical necessity denials are the single most common reason for insurance claim denials, and understanding exactly how insurers define and apply this standard is the key to overturning them.

How Insurers Define Medical Necessity

The insurance industry's definition of medical necessity is deliberately broad and favorable to denial. Most plan documents define medically necessary services as those that:

• Are appropriate for the diagnosis and treatment of a covered condition
• Are consistent with generally accepted standards of medical practice
• Are not primarily for the convenience of the patient or physician
• Could not be omitted without adversely affecting the patient's condition
• Are provided in the most cost-effective setting appropriate for the patient's clinical needs

The last point is critical. Even if a treatment is clinically appropriate, your insurer may deny it as "not medically necessary" because a cheaper alternative exists — even if that alternative is less effective for your specific situation.

How the Insurer's Definition Differs from Your Doctor's

Your doctor's definition of medical necessity centers on one question: what does this patient need to get better? Your insurer's definition centers on a different question: is this the least costly option that meets a minimum standard?

This gap is where most medical necessity disputes live. A physician may determine that a specific branded biologic drug is necessary for a patient with moderate-to-severe rheumatoid arthritis who has failed first-line treatments. The insurer may deny it as not medically necessary because it has not met its step therapy requirements, or because a lower-cost alternative has not been exhausted — even if the physician knows the patient is unlikely to respond to that alternative.

The insurer's determination is also made by a reviewer who has never seen the patient, relying on clinical criteria (often proprietary guidelines like InterQual, MCG, or Milliman Care Guidelines) rather than a direct assessment of the patient's clinical picture.

The Legal Standard Under the ACA

The ACA does not define "medical necessity" for all plans, but it does establish important protections:

• Insurers must have a process for appealing medical necessity denials.
• For non-grandfathered plans, patients have the right to independent External Independent Review: Complete Guide" class="auto-link">external review of medical necessity determinations.
• External review organizations are required to use independent clinical experts and are not bound by the insurer's proprietary criteria.
• Decisions made by external reviewers are binding on the insurer.

ERISA plans (employer-sponsored self-funded plans) are governed by a different standard. Courts typically review ERISA medical necessity denials under an "abuse of discretion" standard — meaning the insurer wins if its determination was not arbitrary, even if the patient's doctor disagrees. This is a difficult standard to overcome, which is why exhausting the internal appeal process thoroughly is critical before considering litigation.

How to Prove Medical Necessity in an Appeal

Proving medical necessity requires building a case that speaks the language insurers respond to — clinical evidence, specific criteria, and documented treatment history.

Step 1: Obtain the denial letter and the criteria used Request the specific clinical criteria (InterQual, MCG, or your insurer's own guidelines) used to deny your claim. Under ACA regulations, you are entitled to this information. Read the criteria carefully to identify exactly which criterion your insurer says you did not meet.

Step 2: Get a detailed letter of medical necessity from your treating physician This is the most important document in your appeal. The letter should:

• State the diagnosis (with ICD-10 code) and clinical history
• Explain why the requested service is necessary for this specific patient
• Address each clinical criterion the insurer cited in the denial
• Reference relevant medical literature and clinical guidelines from specialty societies
• Document prior treatments tried and why they failed

Step 3: Gather clinical literature Peer-reviewed studies, clinical practice guidelines from specialty societies (e.g., American College of Cardiology, American Cancer Society, American Psychiatric Association), and expert consensus statements are powerful evidence. Insurers' own criteria are often derived from this literature — citing the same sources forces them to defend why they apply the criteria differently.

Step 4: Get a second opinion letter if possible A second physician's opinion — especially from a specialist in the relevant field — supporting the medical necessity of the treatment adds significant weight to an appeal.

Step 5: Request external review if the internal appeal fails External reviewers are independent from the insurer and apply objective clinical standards. They overturn medical necessity denials approximately 39–45% of the time, according to analyses of state external review data.

What to Do If This Applies to You

Medical necessity appeals win when they are specific, clinical, and comprehensive. The weakest appeals are one-paragraph letters saying "my doctor says I need this." The strongest appeals address each criterion, cite clinical literature, document treatment history, and make it harder for the insurer to deny again without exposing itself to regulatory scrutiny.

Start with your EOB. Identify the specific denial code. Request the criteria. Then build your appeal around exactly what the insurer said was missing.

Fight Back With ClaimBack

ClaimBack is built around medical necessity appeals. Our system identifies your denial type, helps you gather the right documentation, and generates a structured appeal letter that directly addresses your insurer's specific objections. You do not need a law degree or a medical degree — you need the right approach.

Medical necessity denials are overturned every day. The patients who win are the ones who file detailed, evidence-based appeals — exactly what ClaimBack helps you create.

Start My Free Appeal →

Related Reading

• How to Write an Insurance Appeal Letter
• What Is Medical Necessity?
• Common Reasons Insurance Claims Are Denied