If by theoretically you mean as per the DSM 5 nomenclature; then one approaches a patient in one of two ways: 1) if they have chronic psychosis you first see if they meet the A criterion of schizophrenia. If they do then you look at the other criteria. The D criterion says you have to rule out affective illness. If there has been a mood EPISODE (not just some symptoms), then it is schizophrenia if the total length of all affective episodes is < 50% of the time since they became ill. Following the D criterion basically means that a person cannot have both Bipolar Disorder (or MDD) and schizophrenia. 2) If you have a patient who is presenting with an affective episode, you can look to see if the A criterion psychotic symptoms are present for >2 weeks in the absence of a mood episode and if the mood episodes duration > psychotic symptom duration. If that is the case, it is schizoaffective.
