You're very welcome, gitana1
Your question is an excellent one, and mental health professionals are hesitant to give clear answers to that. As the weblink I provided in earlier post states, there is ambiguity in the differential diagnosis between schizophrenia and bipolar disorders. This is because there is significant overlap in symptoms, treatment, and neuropathophysiology between the two.
E.g., schizophrenia and bipolar disorder share medications. Five atypical antipsychotics originally only approved to treat schizophrenia have now also been approved to treat acute mania. This leads one to believe that these two disorders may share similar neuropathophysiology.
Another example. The positive symptoms of schizophrenia can look like the symptoms in approx. 50% of manic episodes of bipolar patients, particularly those with psychosis, including delusions of grandeur, hallucinations, disorganized speech, paranoia. By the same token, the negative symptoms of schizophrenia can resemble the symptoms of a depressive episode in a bipolar patient -- apathy, extreme emotional withdrawal, lack of affect, low energy, etc.
Furthermore, the two disorders share abnormalities in some of the same neurotransmitter systems. For instance, both depressive episode symptoms of bipolar patients and the negative symptoms of schizophrenia are partially mediated by serotonin. Correspondingly, the positive symptoms of schizophrenia and the manic symptoms of a bipolar patient are mediated in part by excesses of dopamine signalling. Hence, again, the atypical antipsychotics approved for both these disorders work on both the serotonin and the dopamine systems.
To conclude, the diagnosis of either schizophrenia vs. bipolar disorder is chiefly based on the cluster of symptoms presented by the patient to the mental health specialist at a given time or specified period of time. Because these two psychiatric disorders are part of the same cognitive/thought disorder continuum, diagnosis of one vs. the other is not as clear-cut as diagnosing physical illnesses which are quantifiable. For instance, if you have a physical disease such as leukemia, there are physical exams such as blood tests which help determine the exact disease. Such physical tests are not available for mental disorders. Hence the ambiguity. Add the fact that some mental conditions do change over time due to varied external and internal stressors, and the cluster of symptoms that a specialist may have diagnosed as bipolar disorder a few months ago may then changed to appear more like schizophrenia at another period of time. Thus, even more ambiguity.
I hope that answers your question somewhat.