One of the chief challenges many behavioral health clinicians struggle with related to psychotherapy with clients who will be using their insurance is how to define medical necessity. I have had many conversations with clinicians who struggle to sort out what the “insurance company wants”. 
While it seems like this should be a straightforward issue – “why is this treatment necessary?” it can be challenging to coalesce everything in a clear way.
Of course, the point of the biopsychosocial assessment is to gather the information and start building the diagnosis to start the treatment, understanding the psychological symptoms, along with the social components and medical history potentially impacting them. While not a necessity, I tend to think that the most straightforward way to start the assessment that will put you on a good path, a strong Presenting Problem section at the beginning of the assessment can be a helpful first step.
If the Assessment were a story, the Presenting Problem is its thesis. It is the Who, What, Where, How, and Why.
  • · Who is the client?
  • · What are the symptoms they are facing? Not the diagnosis.
  • · Where does this occur?
  • · How does it impact their life?
  • · Why are they coming into treatment?
With any good thesis, this is not a long paragraph, just a few clearly constructed sentences. So, a simple Presenting Problem statement might look like –

Client is a 35-year-old CIS female, single with no current partner, who reports experiencing profound sadness throughout the day, reduced sleep and appetite, and is struggling with remaining focused at work. Client reports feeling tired all the time, is withdrawn from friends, and has decreased her social activities with friends, which have all been occurring for over the past 9 months. While client reports not experiencing suicidality, she wonders what the world would be like if she were no longer in it. Client wants to not be sad all the time and figure out how to re-engage in her life.

It's a few sentences, we’ve pulled a lot of information that came from the assessment and did not identify any diagnosis. Does it list everything that was learned in the assessment? It doesn’t. Just like a thesis, you are not trying to tell the whole story, just create a context. There is nothing related to her unremarkable health history, or the complexities of her relationship with her parents or siblings. Her frustrations with her education or job may play a role in treatment along the way, but it doesn’t need to be here. It doesn’t include clinical details either, like her PHQ-9 score of 10, though its definitely in the assessment. The Presenting Problem is also not simply a restatement of the client’s statement. It is the thesis of the 
What these few sentences do is focus attention on the primary elements which will establish the basis of the medical necessity argument and ultimately the treatment plan. And while the Presenting Problem is most often the first thing on the psychosocial assessment, it really can only be written at the end of the assessment.
Is this all of medical necessity? In most cases there will be more to it, including of course, the diagnosis. In less complex cases, though, the Presenting Problem could be much of what you need to establish the basis of necessity.