A skilled clinical interview is the mainstay of diagnosing borderline personality disorder. This approach should be complemented by knowledge of the DSM criteria and a longitudinal view of the clinical picture. The additional use of assessment instruments can be useful, especially when the diagnosis is unclear. Use of such instruments must be accompanied by clinical judgment.

Certain assessment issues relevant to all personality disorders should be considered when diagnosing borderline personality disorder. For the diagnosis to be made, the personality traits must cause subjective distress or significant impairment in functioning. The traits must also deviate markedly from the culturally expected and accepted range, or norm, and this deviation must be manifested in more than one of the following areas: cognition, affectivity, control over impulses, and ways of relating to others. Therefore, multiple domains of experience and behavior (i.e., cognition, affect, intrapsychic experience, and interpersonal interaction) must be assessed to determine whether borderline traits are distressing or impairing. The clinician should also ascertain that the personality traits are of early onset, pervasive, and enduring; they should not be transient or present in only one situation or in response to only one specific trigger. It is important that borderline personality disorder be assessed as carefully in men as in women.

The ego-syntonicity of the personality traits may complicate the assessment process; the use of multiple sources of information (e.g., medical records and informants who know the patient well) can be particularly helpful in establishing the diagnosis if the patient's self-awareness is limited. Given the high comorbidity of axis I disorders with borderline personality disorder, it is important to do a full axis I evaluation. An attempt should be made to distinguish axis I states (e.g., mood disorder) from borderline personality disorder, which can be a complex process. Useful approaches are to obtain a description of the patient's personality traits and coping styles when prominent axis I symptoms are absent and to use information provided by people who have known the patient without an axis I disorder. If axis I disorders are present, both the axis I disorders and borderline personality disorder should be diagnosed.

Because the personality of children and adolescents is still developing, borderline personality disorder should be diagnosed with care in this age group. Often, the presence of the disorder does not become clear until late adolescence or adulthood.

When assessing a patient with borderline personality disorder, the clinician should carefully look for the presence of risk-taking and impulsive behaviors, mood disturbance and reactivity, risk of suicide, risk of violence to persons or property, substance abuse, the patient's ability to care for himself/herself or others (e.g., children), financial resources, psychosocial stressors, and psychosocial supports (e.g., family and friends).