CO Rev. Stat. §10-16-113(2-3)

(2) Following a denial of a request for benefits or an adverse determination by the carrier, the carrier shall notify the individual in writing.

(3)(a)(I) All denials of requests for reimbursement for medical treatment, standing referrals, or adverse determinations made on the ground that a treatment or covered benefit is not medically necessary, appropriate, effective, or efficient, is not delivered in the appropriate setting or at the appropriate level of care, or is experimental or investigational, must include:

(A) An explanation of the specific medical basis for the denial;

(B) The specific reasons for the denial or adverse determination;

(C) Reference to the specific health coverage plan provisions on which the determination is based;

(D) A description of the carrier’s review procedures and the time limits applicable to such procedures and a statement that the individual has the right to appeal the decision;  and

(E) A description of any additional material or information necessary, if any, for the individual to perfect the request for benefits and an explanation of why the material or information is necessary