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The onset of bipolar disorder may begin with either a depressive or a manic episode. In general, however, patients experiencing both types of episodes are more likely to report depression to a physician.

Bipolar disorder is often under-diagnosed and misdiagnosed. According to the National Mental Health Association (NMHA), up to 80 percent of patients with the condition go undiagnosed or misdiagnosed for up to 10 years. However, there may be an over-representation of bipolar disorder in higher social-economic and education groups. This may be because these groups may be more aware of the disorder and are more likely to acknowledge and accept mental health disorders.

Before bipolar disorder can be diagnosed, a physician must perform a physical examination to rule out other potential causes of the symptoms, such as depression. Substance abuse, many medications or medical conditions (e.g., thyroid disorders, viral infections) can cause symptoms similar to bipolar disorder. Lab tests may also be ordered, particularly to check for thyroid dysfunction, which can make the symptoms of bipolar disorder worse.

The actual diagnosis of bipolar disorder begins with a mental health evaluation performed either by a physician or by referral to a mental health professional. This evaluation includes a complete history of symptoms, with particular attention to their onset, duration and severity. It is also noted whether the patient has had these symptoms before and, if so, whether and how they were treated. The Mood Disorder Questionnaire (MDQ) may be used, as well. This is a set of questions for the patient to answer that helps to determine if that patient has had any previous manic symptoms.

The physician or mental health professional will also ask about alcohol and drug use, whether the patient has thought about death or suicide and whether other family members have had a mood disorder. If the patient has a family history of mood disorders, they will be asked whether and how family members were treated and how effective the treatment was.

Bipolar disorder is generally characterized by many separate episodes of mania or depression. For an episode to be considered a new episode, the following criteria must exist:

• There must be a shift in mood or lapse of time. The patient must display a drastic shift in mood, such as from major depression to mania. A new episode may also be diagnosed if it is separated from the previous episode by at least two months of normal mood.
  
  
• Episodes must occur spontaneously. They cannot be directly caused by a general medical condition or the use of prescription or illegal substances.
  
  
• Psychotic disorders (e.g., schizophrenia) must be ruled out.

Diagnosis of bipolar disorder is largely dependent on symptoms. For instance, if a manic or mixed episode ever occurs, bipolar I disorder is diagnosed .