For outpatient Evaluation and Management (E/M) services, “medical necessity” is ultimately judged against Medicare coverage policy, which is established through National Coverage Determinations (NCDs) and Local Coverage Determinations (LCDs). NCDs set nationwide rules for when specific services are considered reasonable and necessary, while LCDs are developed by Medicare Administrative Contractors to define coverage expectations within their jurisdictions, including indications, frequency limits, required documentation elements, and diagnosis-to-service relationships. Outpatient CDI and coding education emphasizes that correct E/M code selection and documentation must support not only the level of service (MDM/time) but also why the service was needed based on the patient’s condition and the payer’s coverage criteria. Specialty society recommendations and AMA/AHA guidance can inform clinical practice and coding conventions, but they do not define Medicare coverage requirements. Likewise, AHIMA provides professional guidance but does not set payer medical necessity policy. Therefore, the most accurate source defining medical necessity criteria for billing compliance is NCDs and LCDs.