CLINICAL HANDBOOK OF SCHIZOPHRENIA

12. Diagnostic Interviewing 123tual) context by using an interpreter when needed, recognizing that the interviewee’shealth beliefs and health-related behaviors may be very different from those of the interviewer.Guideline 7: Differential DiagnosisThe symptoms of schizophrenia often overlap with those of many other psychiatric disorders;thus, the presence of other syndromes should be assessed and ruled out before thediagnosis of schizophrenia can be made. Schizoaffective and mood disorders are commonlyconfused with schizophrenia, because they are mistakenly thought to simplyinclude both psychotic and affective symptoms (i.e., in bipolar disorder and major depressivedisorder with psychotic features). But it is not the predominance of the psychoticversus the affective component that determines the diagnosis; rather, it is the timing ofpsychotic and affective symptoms. If psychotic symptoms and affective symptoms alwaysoverlap, the person is diagnosed with an affective disorder, whereas if psychotic symptomsare present some of the time, in the absence of an affective syndrome, the personmeets criteria for either schizoaffective disorder or schizophrenia (the former, if the moodsymptoms are prolonged).Recent research has revealed high rates of exposure to trauma and posttraumaticstress disorder (PTSD) comorbidity among people with a severe mental illness such asschizophrenia (Switzer et al., 1999). These findings, and the overlap in symptom presentation,make PTSD a highly relevant disorder when assessing schizophrenia. Dissociativeor intrusive (reexperiencing) symptoms, such as trauma-related auditory phenomena andflashbacks, may be mistakenly interpreted as schizophrenia, so special attention is requiredto rule them out.Substance use disorders such as alcohol dependence or drug abuse can either be adifferential diagnosis or a comorbid disorder of schizophrenia. With respect to differentialdiagnosis, substance use disorders can interfere with a clinician’s ability to diagnoseschizophrenia, and if the substance use is covert, lead to misdiagnosis (Kranzler et al.,1995). Psychoactive substances, such as alcohol, marijuana, cocaine, and amphetamines,can produce symptoms and dysfunction that mimic those found in schizophrenia, such ashallucinations, delusions, and social withdrawal (Schuckit, 1989). The most critical recommendationsfor diagnosing substance abuse in schizophrenia include (1) maintain ahigh index of suspicion of substance abuse, especially if an interviewee has a past historyof substance abuse; (2) use multiple assessment techniques, including self-report instruments,interviews with interviewees, clinician reports, reports of significant others, andbiological assays; and (3) be alert to signs that may be subtle indicators of the presence ofa substance use disorder, such as unexplained symptom relapses, increased familial conflict,money management problems, and depression or suicidality.Many general medical disorders, such as hyperthyroidism, and cognitive disorders,such as dementia of various types, can present with schizophrenia-like symptoms. Inmany of these disorders the cognitive impairments are similar (e.g., in some cases of headinjury). Hence, the differential diagnosis of schizophrenia in relation to these disordersmay be difficult, particularly when past history is not conclusive (e.g., when a first psychoticepisode started after a head injury). Moreover, the impact of comorbidity, such aswhether a head injury that occurred after the onset of schizophrenia is contributing tosymptom severity and cognitive impairment, may be very difficult to determine, becausethe natural course of schizophrenia in itself is not a uniform one. Still, a thorough medicaland psychiatric history is helpful in this respect, as are laboratory tests—blood testsfor hormones and many other factors, brain imaging such as computed tomography and