Major Depressive Disorder
A discrete episode of persistently low mood and/or loss of interest lasting at least two weeks, accompanied by a cluster of physical, cognitive, and behavioral changes severe enough to impair functioning.
Major Depressive Disorder vs. Bipolar I Disorder
MDD is defined purely by depressive episodes; Bipolar I is defined by the presence of at least one manic episode, regardless of whether depressive episodes ever occur.
Onset / developmental pattern
Both often first appear in the teens through twenties; a first presentation that looks like depression can later prove to be bipolar once a manic episode eventually occurs.
Duration / course
A major depressive episode requires two-plus weeks; a manic episode requires one-plus week (or any duration if hospitalization is required).
Associated features
MDD involves low mood/energy/interest; mania involves elevated/expansive/irritable mood, decreased need for sleep, grandiosity, pressured speech, and increased goal-directed activity -- essentially the opposite symptom profile.
Functional impairment
Depressive impairment tends to look like withdrawal and shutdown; manic impairment tends to look like impulsive, consequential overactivity (financial, legal, occupational risks).
Rule-outs / exclusions
The single decisive exclusion question for MDD is: has the person ever had a manic or hypomanic episode? If yes, the diagnosis moves to the bipolar spectrum even if depressive episodes dominate the clinical picture.
The differential question
Never diagnose MDD without asking about any lifetime history of unusually elevated mood, decreased sleep need without fatigue, or impulsive high-risk behavior lasting days to weeks -- a single missed manic history is a common differential error.
Common student confusion
Students sometimes think "mood swings" (rapid, moment-to-moment shifts) indicate bipolar disorder. Genuine manic/depressive episodes are typically sustained over days to weeks, not minutes to hours.
Case clues
A history of any period of significantly reduced sleep need without tiredness, racing thoughts, or uncharacteristic impulsive spending/decisions is the key clue pointing away from pure MDD toward Bipolar I.
Read each disorder in full
Bipolar I Disorder
A condition defined by the occurrence of at least one manic episode -- a distinct period of abnormally and persistently elevated, expansive, or irritable mood with increased energy, lasting at least one week (or requiring hospitalization).