Physiological (body) illnesses, substance abuse, and other mental disorders can mimic, or look like, bipolar mood symptoms. Because of this, it is important that any clinician attempting to diagnose bipolar disorder be careful to rule out alternative causes for observed symptoms.
A definitive diagnosis of bipolar disorder can only be made after a patient has been medically screened and cleared for other medical conditions which might contradict bipolar disorder diagnosis, has been sober for a long enough time for any suspected substances that might influence mood to have cleared the body, and is known not to have other mental disorders which would contradict bipolar disorder. This process takes some time to complete.
Though a provisional diagnosis may be made rather quickly, so as to facilitate rapid treatment, it is common for doctors to hold off making a final and definitive bipolar disorder diagnosis until they have had time to observe a patient over a period of several months, and conduct a review of any past hospitalizations and treatments which may have occurred.