Bipolar I is a mood disorder in which psychotic-like features, if present, occur only during mood episodes; schizophrenia is a primary psychotic disorder in which delusions/hallucinations/disorganization persist independent of mood state.
Both often first appear in the late teens to early adulthood, which is part of why the two can be confused at first presentation.
A manic episode requires one-plus week; schizophrenia requires continuous signs for six-plus months including at least a month of active symptoms.
Mania: elevated/expansive/irritable mood, decreased sleep need, grandiosity, pressured speech. Schizophrenia: negative symptoms (flattened affect, avolition), disorganized speech/behavior, and fixed delusions/hallucinations not tied to mood.
Bipolar I impairment often clusters around discrete episodes with fuller functioning between them; schizophrenia typically shows a marked, sustained decline in baseline functioning, including between acute exacerbations.
The key exclusion question for schizophrenia is whether any psychotic-like symptoms occur exclusively during mood episodes (favors bipolar disorder with psychotic features) or persist independent of mood (favors schizophrenia).
Ask: do the unusual beliefs/perceptions only ever show up during a period of clearly elevated or depressed mood, or are they present even when mood is otherwise stable? The former favors a mood disorder; the latter favors a primary psychotic disorder.
Grandiosity during mania (e.g., inflated self-belief while energized and sleepless) is sometimes mistaken for a fixed delusion; context (does it resolve as the mood episode resolves?) matters for telling the two apart.
Fixed false beliefs, flattened emotional expression, and disorganized speech present even during calm, non-elevated periods point toward schizophrenia rather than bipolar 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).
A disorder involving at least two of five core symptom domains (delusions, hallucinations, disorganized speech, grossly disorganized/catatonic behavior, negative symptoms), with continuous signs persisting for at least six months and significant functional decline.