Criteria
Schizophrenia is diagnosed based on criteria in either the Diagnostic and Statistical Manual of Mental Disorders (DSM) published by the American Psychiatric Association or the International Statistical Classification of Diseases and Related Health Problems (ICD) published by the World Health Organization (WHO). These criteria use the self-reported experiences of the person and reported abnormalities in behavior, followed by a psychiatric assessment. The mental status examination is an important part of the assessment. An established tool for assessing the severity of positive and negative symptoms is the Positive and Negative Syndrome Scale (PANSS). This has been seen to have shortcomings relating to negative symptoms, and other scales – the Clinical Assessment Interview for Negative Symptoms (CAINS), and the Brief Negative Symptoms Scale (BNSS) have been introduced. The DSM-5, published in 2013, gives a Scale to Assess the Severity of Symptom Dimensions outlining eight dimensions of symptoms.

DSM-5 states that to be diagnosed with schizophrenia, two diagnostic criteria have to be met over the period of one month, with a significant impact on social or occupational functioning for at least six months. One of the symptoms needs to be either delusions, hallucinations, or disorganized speech. A second symptom could be one of the negative symptoms, or severely disorganized or catatonic behaviour. A different diagnosis of schizophreniform disorder can be made before the six months needed for the diagnosis of schizophrenia.

In Australia the guideline for diagnosis is for six months or more with symptoms severe enough to affect ordinary functioning. In the UK diagnosis is based on having the symptoms for most of the time for one month, with symptoms that significantly affect the ability to work, study, or to carry on ordinary daily living, and with other similar conditions ruled out.

The ICD criteria are typically used in European countries; the DSM criteria are used predominantly in the United States and Canada, and are prevailing in research studies. In practice, agreement between the two systems is high. The current proposal for the ICD-11 criteria for schizophrenia recommends adding self-disorder as a symptom.

A major unresolved difference between the two diagnostic systems is that of the requirement in DSM of an impaired functional outcome. WHO for ICD argues that not all people with schizophrenia have functional deficits and so these are not specific for the diagnosis.

Comorbidities
Many people with schizophrenia may have one or more other mental disorders, such as panic disorder, obsessive–compulsive disorder, or substance use disorder. These are separate disorders that require treatment. When comorbid with schizophrenia, substance use disorder and antisocial personality disorder both increase the risk for violence. Comorbid substance use disorder also increases risk for suicide.

Sleep disorders often co-occur with schizophrenia, and may be an early sign of relapse. Sleep disorders are linked with positive symptoms such as disorganized thinking and can adversely affect cortical plasticity and cognition. The consolidation of memories is disrupted in sleep disorders. They are associated with severity of illness, a poor prognosis, and poor quality of life. Sleep onset and maintenance insomnia is a common symptom, regardless of whether treatment has been received or not. Genetic variations have been found associated with these conditions involving the circadian rhythm, dopamine and histamine metabolism, and signal transduction.