Overview
Medical Necessity is the contractual and regulatory standard every payer applies to decide whether it will pay for a given service. Medicare's statutory definition, at Section 1862(a)(1)(A) of the Social Security Act, excludes items and services "not reasonable and necessary for the diagnosis or treatment of illness or injury." Private payers have their own definitions, virtually all of which require that the service be consistent with generally accepted standards of medical practice, appropriate for the diagnosis, and not more extensive than necessary to treat the condition.
Concretely, medical-necessity determinations are made against published clinical policies: Medicare Local Coverage Determinations (LCDs) issued by MAC contractors, Medicare National Coverage Determinations (NCDs) issued by CMS, and each commercial payer's published medical policies. These documents specify the covered indications, required clinical evidence, documentation requirements, and sometimes the specific CPT/HCPCS codes considered necessary. When a claim's linked ICD-10 diagnoses do not support the billed CPT under the applicable policy, the payer denies with CARC 50 ("These are non-covered services because this is not deemed a 'medical necessity' by the payer").
Medical-necessity denials rank among the hardest to recover because the remediation requires clinical documentation to support the claim retrospectively. The appeal packet typically includes the physician's note documenting the clinical rationale, relevant imaging or lab results, prior conservative-therapy attempts if required by policy, and an attestation letter from the treating provider. Appeal success rates depend heavily on the strength of the original documentation — if the clinical note does not establish necessity at the time of service, it is difficult to construct an appeal packet that does. This is why Clinical Documentation Improvement (CDI) programs prioritize medical-necessity language in the note: a concise paragraph tying the ordered service to the patient's diagnosis, clinical findings, and evidence-based criteria materially improves both first-pass approval and appeal success.
A frequent confusion is conflating medical necessity with prior authorization. They are related but distinct. Prior authorization is the payer's advance-review process — completed before the service is rendered, resulting in an authorization number and sometimes a modified approval. Medical necessity is the adjudication standard applied at claim review, whether or not prior authorization was obtained. A service can have prior authorization and still be denied for medical necessity if the billed CPT does not align with the authorized procedure or if downstream documentation fails to support it. Conversely a service without prior authorization can be paid on medical necessity if the plan does not require PA.
Operationally, a strong medical-necessity program prevents denials rather than fighting them. Order-entry decision support surfaces the applicable LCD or payer policy at the point of ordering, prompts the clinician to document the supporting diagnosis, and warns when the ordered combination is likely to fail policy. Clinical-documentation specialists review high-dollar orders before claim submission. Payer-specific medical-policy repositories, refreshed quarterly, power both the ordering-time checks and the claim-scrubbing edits.
Industry benchmark
CMS policy is published at the CMS Medicare Coverage Database (LCDs and NCDs); each commercial payer publishes its clinical policies on its provider portal. HFMA and industry data identify medical-necessity denials as consistently among the top three CARC categories in hospital revenue cycle, alongside authorization and eligibility denials.
Worked example
A patient presents with low back pain. The ordering physician requests a lumbar MRI without first documenting a trial of conservative therapy, which the payer's medical policy requires before imaging. The claim denies with CARC 50. The appeal succeeds after the physician supplements the documentation with a retrospective letter detailing six weeks of physical therapy and NSAID trial that were in the prior visit notes but not referenced in the order.
Frequently asked questions — Medical Necessity
What is the difference between medical necessity and prior authorization?
Prior authorization is an advance-review workflow; medical necessity is the adjudication standard applied at claim review. A service can have prior authorization and still be denied for medical necessity if documentation does not support the billed CPT, and a service without PA can be paid on medical necessity when PA is not required.
What is CARC 50?
Claim Adjustment Reason Code 50 is the standard denial code for services not deemed medically necessary. It is a subjective payer determination based on the applicable clinical policy, and appeals typically require supplemental clinical documentation and sometimes a peer-to-peer review.
Where are Medicare medical-necessity policies published?
CMS publishes National Coverage Determinations (NCDs) and Medicare Administrative Contractors publish Local Coverage Determinations (LCDs) in the CMS Medicare Coverage Database. Commercial payers publish their policies on their provider portals; each plan's policy library is the authoritative source for that plan.
How can we prevent medical-necessity denials?
Order-entry decision support that surfaces applicable LCDs or payer policies, clinical-documentation improvement that strengthens the rationale language in the note, and payer-specific scrubbing rules at claim submission together prevent most medical-necessity denials. The most effective intervention is documentation at the time of order rather than after the denial.
Disclaimer
This glossary entry is operational reference for revenue-cycle and medical-billing professionals. It is not legal, clinical, or contractual advice. Industry benchmarks cite named public sources where available; always verify against the current guidance from the authority body before relying on a number in a contract, policy, or compliance filing.