My advice regarding medical necessity has always been that this is something that is outside the scope of a coder's training and incorporating medical necessity into a coding process should only be done with great care and with the participation of a provider or trained clinician in your organization.
Coders are trained to assign codes based on documentation, and when it comes to using medical necessity as a criteria, you are basically authorizing coders to 'override' documentation - in other words, to disregard certain components of the documentation and being not a necessary part of the patient's care and therefore not eligible to be counted toward the code assignment. In the hands of a coder without clinical training, this becomes a very subjective exercise leading to coder's making decisions such as 'this patient doesn't seem very sick'. But coders are not trained in medical standards of care and can't know what particular symptoms or conditions require a provider to do a more thorough exam or spend more time ruling out potential issues, which is why it's so important to always do this under the guidance of someone with the appropriate training. Medical necessity is an important consideration and documentation does need to support the necessity of what is billed, but in my experience, when coders start making independent decisions about medical necessity and t down-code services without their providers' involvement, this can cost a practice significant revenue loss and also eventually lead to an unpleasant confrontation with providers.
