Medical Necessity
Definition
Medical necessity is the standard payers use to determine whether a healthcare service is covered. A service is medically necessary when it is required to diagnose or treat a medical condition, is consistent with accepted clinical standards, and is not primarily for the convenience of the patient or provider. Payers deny claims under CO-11 when they determine the service does not meet medical necessity criteria.
Why Medical Necessity Matters
Medical necessity is the single most important concept in medical billing reimbursement. Every denied claim for "not medically necessary" traces back to documentation that failed to demonstrate why the service was needed for this patient at this time. The diagnosis code (ICD-10), clinical notes, and supporting documentation must collectively prove medical necessity. See medical necessity regulations.
How Medical Necessity Is Determined
Payers apply clinical criteria (InterQual, Milliman Care Guidelines, or their own proprietary criteria) to evaluate whether the service meets coverage standards. The provider's documentation must show: the patient's diagnosis, the clinical findings supporting the diagnosis, why this specific treatment is needed, and what alternatives were considered. For prior authorization requests, medical necessity documentation is reviewed before the service. For post-service claims, payers conduct retrospective medical necessity review.
Related Terms
Prior authorization — pre-service medical necessity review. Utilization review — the payer's process for evaluating medical necessity. Appeal process — how to contest a medical necessity denial. ICD-10 code — the diagnosis that establishes the clinical basis for medical necessity.
Common Questions
Who decides what is medically necessary?
The payer makes the coverage determination based on their clinical criteria. Providers can appeal by submitting clinical documentation, peer-reviewed literature, and specialist opinions supporting the medical necessity of the service. External review by an independent physician is available as a final appeal step in most states.
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This glossary is for informational purposes. Consult official billing guidelines and payer policies for definitive definitions. Last updated: 2026-04-06.