Start with the letter, because it tells you why you were denied and how long you have to respond. Denial notices may use terms such as “adverse benefit determination.” For plans subject to ACA claims-and-appeals rules, the notice generally must explain the reason for the denial and describe your appeal rights. The reason is your roadmap: a coding or paperwork denial is fixed very differently from a “not medically necessary” denial. Identify which one you are dealing with before writing anything.