This question recently appeared on CMS's frequently asked questions (FAQs) section, here. And, this is CMS's response:
A provider/supplier cannot bill the beneficiary for services denied due to an MUE [(medically unlikely edit)]. An MUE denial is an initial determination based on a coding denial, not a medical necessity denial. By statute an ABN may be applied only if the initial determination on a claim results in a denial due to medical necessity. If a provider appeals an MUE denial and some UOS are denied as not medically necessary, the provider should NOT apply an ABN to bill the beneficiary. An appeal is not an initial determination, and by statute the ABN provision only applies to the initial determination.
Subscribers can view our latest MUE coverage here and here.