A personality disorder impacts the way the patient perceives himself, events and others (cognition), his/her range, intensity, appropriateness, and lability of mood (affectivity) and it influences interpersonal functioning and impulse control (behavior).
Knowing if a personality disorder, abnormal functioning, has influence on patient behavior enables us to understand, predict and ideally control any inappropriate behaviors that arise, including non-compliance.
Thus, if we know a patient is a (socially) avoidant personality, a paranoid personality, a dependent personality, a negativistic personality or an anti-social personality, or even an amalgam of several personality disorders, we are able to predict how the patient will respond to the stressors and demands of his/her illness or injury.
Conversely, if we do not have a formal measure of their personality functioning, we jump through hoops, trying to adjust our own behaviors to suit them, a process that is destined to failure.
The earlier we assess personality functioning, and abnormal variants, the sooner we have reasonable understanding and control over the health management process.